Conditions For The Acceptance of Actions
Source : Six Conditions of Ibadah , By Sheikh Muhammad bin Salih al-'Uthaymeen
Imam Muhammad Ibn Salih al-'Uthaymeen (d.1421H) said, "Sincerity (Ikhlas) towards Allah means that the person desires
by his worship closeness to Allah the Exalted and reaching His Abode of Nobility (i.e. Paradise), such that the servant must be
sincere towards Allah the Exalted in his intended purpose. He must be sincere towards Allah the Exalted in his love.
He must be sincere towards Allah in his exaltation. He must be sincere to Allah the Exalted outwardly and inwardly.
He does not seek anything with his worship, except for the Face of Allah the Exalted and reaching His Abode of Nobility." 
Stated Al-'Allamah Muhammad Ibn Salih al-'Uthaymeen (d.1421H):
"And in order that it will be known - O brothers - that Al-Mutaba'ah cannot be actualized, except when the action is in agreement with the Shari'ah in six affairs:
The First: As-Sabab (reason, motive)
So when the person worships Allah with act of worship that is accompanied by a motive that is not legislated by the Shari'ah,
then it is an innovation that is rejected back to its doer. An example of that is when some of the people commemorate
the twenty-seventh night of Rajab under the pretence that it was the night in which the Messenger of Allah (Peace and blessings be upon him) ascended.
So tahajjud (the night Prayer) is worship. However, when it is accompanied with this motive, it becomes an innovation.
This is because this action of worship is built upon a motive that has not been confirmed in the Shari'ah.
And this description - the conformity of the worship to the Shari'ah in motive -
is an important affair that clarifies many innovations from amongst those that are thought to be from the Sunnah, yet they are not from the Sunnah.
The Second: Al-Jinss (type)
so it is inevitable that the act of worship be in conformity to the Shari'ah in its Jins (type).
So if the person worships with an act of worship whose type is not legislated, then it is not accepted.
An example of that is if a man were to slaughter a horse, then this sacrifice would not be correct,
because it has opposed the Shari'ah in the type. So the blood sacrifices cannot be done, except with cattle, grazing livestock, camels, cows and sheep.
The Third: Al-Qadr (quantity)
so if the person increases in the Prayer that is obligatory, then we say that this is an innovation that is not accepted,
because it opposes the Shari'ah in quantity. With all the more reason, if the person prays the noon Prayer for example as five units,
then his Prayer is not correct by agreement.
The Fourth: Al-Kayfiyyah (manner, mode)
so if a man performs ablution, and he begins with washing the feet, then he wipes his head, then he washes his hands,
then his face, then we say that his ablution is null and void, because it opposes the Shari'ah in manner.
The Fifth: Az-Zaman (time)
so if the person slaughters in the first days of Dhul-Hijjah, then his blood sacrifice will not be accepted due it opposing the Shari'ah in time.
And I have heard that some of the people slaughter sheep during the month of Ramadan in order to draw closer to Allah with blood sacrifice.
So this action in this manner is an innovation, because there is nothing with regards to drawing closer to Allah with slaughtering,
except for blood sacrifice, gifts and the 'Aqiqah. As for slaughtering in Ramadan along with the belief that one will attain the reward of slaughtering,
such as the blood sacrifice in 'Eid Al-Adha, or slaughtering for the sake of meat, then this is permissible.
The Sixth: Al-Makan (place)
So if a man performs I'tikaf in other than a mosque, then his I'tikaf is not correct.
And that is because the I'tikaf cannot occur, except in the mosques. And if a woman says, 'I wish to perform I'tikaf in the Musalla (place of Prayer) in the house,'
then her I'tikaf is not correct due to opposition of the Shari'ah in place. And from the examples of that is if a man desires to perform Tawaf.
So he finds that the area of Tawaf has become too confined and he finds that whatever is around it has become too confined.
So he starts to perform Tawaf from behind the mosque, then his Tawaf will not be correct, because the place of the Tawaf is the house.
So the act of worship cannot be considered a righteous action, except if two conditions are fulfilled with regards to it,
the first is Al-Ikhlas (sincerity) and the second is al-Mutaba'ah (following the Prophet).
And Al-Mutaba'ah cannot be fulfilled, except with the six affairs that have been previously mentioned." 
: Refer to Majmu' Al-Fatawa (7/112) of Ibnul-'Uthaymeen.
: Refer to Majmu' Al-Fatawa (5/253-254) of Ibnul-'Uthaymeen.