Sources of the Deen
The Book of Allaah, the Most High, is their manual (dustoor) and their law then the Sunnah after  the Book of Allaah. The Sunnah is the most blessed of sources and the best of sciences, the most beneficial of all, in the Deen and Dunyah (world), after the Book of Allaah, the One free of all defects. It is like the meadows and gardens, you will find in it all goodness and righteousness. After the Sunnah, their manual is that which the Salaf of this Ummah and it's A'immah have agreed upon.
The Salaf as-Saalih are also: The Best Generation, about which the Messenger of Allaah sallallaahu 'alayhi wa sallam said in a hadeeth:
"The best of the people are my generation, then those after them, then those after them ... "
and he sallallaahu 'alayhi wa sallam said: " .. Then there will come a people giving witness when they are not asked to give witness. They will be dishonest and not be trusted. They will take vows but will not fulfill them. Fatness will appear amongst them."
From their beliefs ...
Thus the fundamentals of the Religion (Usool ud-Deen) which were adhered to by those that preceded from the leaders (a'immah) of the Deen, the scholars of the Muslims and the Salaf as-Saalih and what they called people to are:
That they believe in the Book and Sunnah;  the general and the detailed aspects of it. They attest to the oneness of Allaah the Mighty and Majestic, and attest to the Messengership of Muhammad sallallaahu 'alayhi wa sallam.
They know their Lord by the attributes (sifaat) that His revelation has spoken of, or are attested to by the Messenger of Allaah sallallaahu 'alayhi wa sallam from that which is found in authentic ahadeeth, narrated from him by just and reliable narrators.
They affirm for Allaah, the Most High, that which He affirmed for Himself in His Book, or upon the tongue of His Messenger sallallaahu 'alayhi wa sallam without making Tashbeeh  (resembling) to His creation, without Tahreef (changing), without Tabdeel and without Tamtheel. 
Allaah, the Most High, says:
There is nothing like unto Him, and He is the all Hearer, all Seer.
Imaam az-Zuhree  said: "Upon Allaah is the bayaan (explaining), upon the Messenger is the balaagh (conveying) and upon us is tasleem (willingly accepting)."
[This is reported by al-Bukhaaree in mu'allaq form (13/503), by Ibn Abee Aasim in Kitaab al-Adaab and in Kitaab az-Zuhd (71) and by Aboo Nu'aym in Hilyah (3/179)]
Imaam Sufyaan ibn 'Uyaynah  said: "All that Allaah has described Himself with in His Book, then its tafseer (explanation) is its reciting and keeping silent about it."
Imaam ash-Shaafi'ee  said: "I believe in Allaah, and that which has come from Allaah, upon the intended meaning of Allaah. I believe in the Messenger of Allaah sallallaahu 'alayhi wa sallam and that which has come from the Messenger of Allaah upon the intended meaning of the Messenger of Allaah sallallaahu 'alayhi wa sallam."
The Salaf and leaders of the khalaf (those later generations after the Salaf) many Allaah be pleased with them, proceeded along this way. They are all agreed upon, affirming, asserting and confirming that which is found in the Book of Allaah, the Most High, and the Sunnah of His Messenger sallallaahu 'alayhi wa sallam, regarding Allaah's attribtutes without subjecting them to interpretation (ta'weel).
Verily we have been ordered to follow their way and to be guided by their light. The Messenger of Allaah sallallaahu 'alayhi wa sallam has warned us of newly invented matters, and informed us that they are from misguidance. He said in a hadeeth:
"Adhere to my Sunnah (way) and the the Sunnah of the rightly guided Khulafaa'. Bite on to it with your molar teeth, and beware of newly invented matters, for verily every bid'ah (innovation) is a going astray."
Its reference (takhreej) has preceded. (see footnote no. 6).
'Abdullaah bin Mas'ood  radiallaahu 'anhumaa said: "Follow and do not innovate for verily you have been sufficed."
'Umar bin 'Abdul-'Azeez  said: "Do not go beyond where they stopped. For verily they stopped upon knowledge and with a perceiving view sufficed."
Imaam al-Awzaa'ee  said: "Stick to the footsteps of the Salaf, even if people abandon you. Beware of the views of men, even if they beautify it for you with words."
From the 'aqeedah  (beliefs) of the Salaf as-Saalih is their saying: Eemaan is a saying of the tongue, an action by the limbs and a strong belief in the heart. Eemaan increases with obedience to Allaah and decreases with disobedience to Allaah.
From the beliefs of the Salaf is that good and evil is by the Qadaa (decree) of Allaah the Most High and His Qadr (pre-estimation). However evil is not ordered by Allaah, as some of them say: "All of it is ordered by Allaah," since Allaah has ordered good and prohibited evil. He did not order us with abominable acts, indeed He prohibited us from them. The human is not forced, he chooses his acts and beliefs. He deserves punishment or reward, depending upon his choice and he is the chooser in (doing or leaving) the ordered and prohibited things.
Allaah, the Most High, says:
Then whosoever wills let him believe, and whosoever wills let him disbelieve.
From the beliefs of the Salaf is that they do not make takfeer  of anyone of the Muslim due to a sin, even if it is from the major sins. Except if he denies (rejects) a thing that is known in the religion by necessity and is known by the scholars as well as the generality of people, and is based firmly upon the Book and the Sunnah and Consensus (Ijmaa') of the Salaf of this Ummah and its leader.
From the beliefs of the Salaf as-Saalih is that they worship Allaah, the Most High, and do not associate anything with Him. Thus they do not ask any one except Allaah, the Most High, of a need no one else can satisfy. They do not seek help (in which no one can help) from anyone except Allaah, the One free of all defects. They do not call upon anyone for aid (for an immediate need that no one can fulfil) except upon Allaah. They do not make tawassul (seeking nearness) to Allaah except by obeying Him, worshipping Him and doing good / righteous deeds.
This being taken from His saying:
O you who believe! Do your duty to Allaah and fear Him, seek the means of approach to Him.
That is, draw close to Allaah by obedience to Him and by worship of Him.
From the beliefs of the Salaf as-Saalih is that Prayer (Salaat) behind all righteous people and sinners is permissible if the external nature of the prayer is correct. 
We do not certify with certainty for anyone, whoever he may be, of being in Paradise or in Hell; except whoever the Messenger of Allaah sallallaahu 'alayhi wa sallam testified for. However we hope Paradise for the righteous and fear Hell for the sinful.
We testify for the ten people given the glad tidings of Jannah (Paradise), that they will be in Jannah (Paradise); just as the Prophet sallallaahu 'alayhi wa sallam testified this for them. We testify, as being in Jannah, for everyone that the Prophet sallallaahu 'alayhi wa sallam testifies this for them. This is because the Prophet does not speak of his own desire. Indeed it is nothing but revelation revealed to him. We take the Companions of Allaah's Messenger sallallaahu 'alayhi wa sallam as awliyaa (friends). We refrain from showing their defects  and that which happened between them. Their affair is with their Lord. We do not abuse anyone of the Companions. This is taken from the Prophet's sallallaahu 'alayhi wa sallam saying:
"Let none of you abuse (slander) my Companions. For by Him in whose Hand is my life, if one of you spent (in the way of Allaah) the equivalent of mount Uhud in gold it would not reach a handful nor half a handful of what they spent"
The Companions are not infallible from error. Infallibility is for Allaah, the Most High, and for His Messenger sallallaahu 'alayhi wa sallam in conveying the message. Allaah, the Most High, has protected the collective Ummah from mistakes (and not
individuals) as the Prophet sallallaahu 'alayhi wa sallam said:
"Indeed Allaah will never unite this Ummah upon misguidance and the Hand of Allaah is upon the Jamaa'ah."
We are pleased with the wives of the Messenger of Allaah sallallaahu 'alayhi wa sallam, they are the mothers of the believers and we believe that they are pure and innocent from all evil.
From the beliefs of the Pious Predecessors is that they do not make it obligatory upon any Muslim to restrict himself to a specific madhhab.  He can move from one madhhab to another, due to the strength of the evidence (in a particular point). The layman has no madhhab. Rather his madhhab is the madhhab of his mufti (scholar who gives religious verdicts).  If a student of knowledge (taalibul-'ilm) has the ability to recognise the proofs and evidences of the Imaams, he should act by it, moving from a madhhab of an Imaam, in any issue, to the madhhab of another Imaam, due to the strength of the evidence, and the more correct understanding, in an issue. Thus he becomes a muttabi' (a follower) and not a mujtahid. [A mujtahid is one who is qualified to extract rulings from the Qur'aan and Sunnah
From the beliefs of the Salaf as-Saalih is that the four Companions: Aboo Bakr, 'Umar, 'Uthmaan and 'Alee radiallaahu 'anhum are the rightly guided Khulafaa' and with them existed the Khilaafatun-Nabuwwah (Khilaafah upon the Prophetic Way) lasting for thirty years including the Khilaafah (ruling period) of Hasan radiallaahu 'anhu. This is taken from the saying of the Prophet sallallaahu 'alayhi wa sallam:
"The Khilaafah in my Ummah is for thirty years then after it is kingship."
From the beliefs of the Salaf as-Saalih is that it is obligatory to have eemaan (faith) in all that the Qur'aan has come with, and what Allaah, the Most High, has ordered us with, and leaving all that Allaah, the Most High, has prohibited us from, the general and the detailed. We believe in all that the Prophet sallallaahu 'alayhi wa sallam has informed us of, when its transmission from him is authentic, in that which we witness or do not witness, regardless of whether we comprehend it, or are ignorant of it or we have not come across the reality of its meaning.
We carry out the orders of Allaah, the Most High, and the orders of his Messenger sallallaahu 'alayhi wa sallam. We refrain from what Allaah, the Most High, has prohibited us from and what the Messenger of Allaah sallallaahu 'alayhi wa sallam prohibited us from. We stop at the hudood (boundaries) of the Book of Allaah, and the Sunnah of the Messenger of Allaah 'Umar ibn 'Abdul-'Azeez said:
"The Messenger of Allaah sallallaahu 'alayhi wa sallam and the leaders of the Muslims after him set examples (Sunnah), the taking of which is holding on to the Book of Allaah, the Most High, and strength upon the Deen of Allaah. It is not for any one to change or alter it, nor to look at a matter in opposition to it. Whoever is guided by it then he is truly guide, and whoever seeks help by it then he is truly helped. Whoever leaves it and follows a way other than the way of the believers, Allaah will turn him to what he has chosen and burn him in hell what an evil abode!"
This is confirmed by the saying of Allaah's Messenger sallallaahu 'alayhi wa sallam:
" ... and beware of newly invented matters, for verily every bid'ah is a going astray."
This hadeeth is a great principle from the principles of the Deen and it resembles another saying of the Prophet sallallaahu 'alayhi wa sallam:
"Whoever invents into this Deen of ours that which is not from it, then it is rejected."
In it is a warning from following newly invented matters in the Deen and worship. The meaning of bid'ah is that which has been invented and has no root in the Sharee'ah pointing to it. As for what has a root in the Sharee'ah pointing to it then it is not legally (technically) a bid'ah; even though linguistically the term bid'ah is may be applied to it. Thus whenever something new is introduced into the Deen, having no root (in the Deen) to return it to, then it is misguidance - deviation. The Deen is free of it, regardless of whether it is connected to issues of belief, actions or sayings.
As for the istihsaan (approval and condoning) of bid'ah, which occurred in the words of the Salaf, then that is in relation to bid'ah al-logawee (linguistic use of bid'ah) and not in the legal (technical) use of the term bid'ah. From these sayings is the saying of 'Umar bin al-Khattab, when he gathered the people in Qiyaam-ur-Ramadaan - Taraaweeh prayers - behind one Imaam in the masjid. He went out and say them praying and said: "What a wonderful bid'ah."
This has a root in the Sharee'ah. Verily the Messenger of Allaah sallallaahu 'alayhi wa sallam prayed it (this prayer) in congregation in the masjid and then left it, fearing that it might become obligatory upon his Ummah and that they would be unable to perform it. Indeed they became free of this fear after the death of the Prophet sallallaahu 'alayhi wa sallam. So 'Umar radiallaahu 'anhu revived it. As for that which is a matter confirmed, in worship, then it is not permissible to make additions to it.
So, for example the adhaan, remains the form in which it was legislated, without addition or subtraction. The Prayer remains upon the manner it was legislated as the Messenger of Allaah sallallaahu 'alayhi wa sallam said:
"Pray as you have seen me praying."
This is an authentic hadeeth, reported by al-Bukhaaree in his Saheeh.
Hajj remains upon the manner in which it was legislated, because the Messenger of Allaah sallallaahu 'alayhi wa sallam said:
"Take from me your rites (of Hajj)."
[Reported by Muslim (no.1297) in the Book of Hajj]
Indeed the Muslims have done things that were not present in the time of the Messenger of Allaah sallallaahu 'alayhi wa sallam, due to them being daroorah (necessities) in preserving Islaam. Indeed they have permitted it and kept quiet about it. Like the gathering of the people to one mushaf (that which the Qur'aan is written upon), by 'Uthmaan ibn 'Affan radiallaahu 'anhu, fearing the splitting of the Ummah. Indeed it was approved of by the Companions radiallaahu 'anhum and that was for a maslaha (benefit).
The example of writing down Prophetic traditions (hadeeth) fearing its loss due to the death of its people. The writing of Tafseer of the Qur'aan and the Hadeeth. The compilation of 'Ilm-an-Nahw (science of grammar) to safeguard the Arabic language, which is a means of understanding Islaam. The formation of 'Ilm-al-Mustalah (science of hadeeth). Thus, there are permissible to safeguard the Islaamic Sharee'ah. Verily Allaah, the Most High, has taken the responsibility of safeguarding His Law - from His saying:
Indeed it is We who have sent down the Dhikr (Qur'aan) and surely We will guard it (from corruption).
The Messenger of Allaah sallallaahu 'alayhi wa sallam said:
"This ilm (Deen) will be carried by the trustworthy ones of each generation. Negating from it the tahreef (alterations) of the ones going beyond bounds, the false assumptions of the liars, and the ta'weel (false interpretations) of the ignorant."
This hadeeth is hasan due to all its chains and shawaahids (supporting narrations).
This is the belief ('aqeedah) - of the first group of this Ummah (i.e. the Companions) radiallaahu 'anhum and it is a pure belief like the purity of fresh drinking water, strong as the firmly fixed mountains, firm as the firmed of hand holds. It is a flawless 'Aqeedah, a straight way, upon the methodology of the Book ad the Sunnah and the sayings of the Salaf of this Ummah and its A'immah (leaders). It is the way which revived the hearts of the pioneers of this Ummah.