Brother Zafran, with respect what Islamirama meant by source and reference is evidence from Quran or Sunnah, not some words between quotation marks.
Islamic rulings must all be derived from Quran and Sunnah and have evidence showing them, and scholars are only researchers who studied them, authenticated the narrations, and learned the linkages, and therefore should always demonstrate such evidence when talking about rulings. That way everyone insha Allah will actually learn, not just hear sentences and then guess whether it is a correct or incorrect opinion.
This matter has degressed into whether prayer is kufr Akbar or kufr Asghar, and that is not what the subject originally was about so I'll just point out quickly a couple of pieces of information:
Minority of scholars including early Hanafi's said that whatever makes a person a kafir, surely is made a muslim again with a declaration that "There is no god but Allah..." and since the person declares it and believes in it then he is not completely out of the fold of Islam. And regarded the kufr in the hadiths (already brought by Islamirama) to be referring to kufr asghar.
The vast majority of scholars regard this though to be kufr akbar taking one out of the fold of Islam. It was also documented that the salaf (companions and tabe'in) have not agreed on anything the neglect of which was outright kufr and apostacy, EXCEPT prayer, stating unanimously that whomever doesn't pray was a non-muslim and was not buried with muslims. Also there is reference that the shariah ruling is that the judge may ask for someone claiming to be muslim but neglecting all prayers to be brought before him, and order him with his authority to pray or be declared an apostate. If the man still defiantly rejects prayer, that is when the judgement of apostacy is passed.
Whether such a person will go to Hell forever or not however is an entirely different thing, and we have no knowledge and dare not trespass upon Allah's judgement or Mercy. There are proofs in Sunnah that show people who did not pray all their lives were inspired to pray mioments before their death and we were told they went to heaven (such as the story of the two brothers). There is the companion who declared Islam and did not pray a single prayer but went out and died and was regarded a muslim and buried with muslims. So we know the ruling, but we do not pass the Hell or Paradise judgement on any specific person as we do not know what Allah will judge him by.
Now this hadith of the prophet is clear and that takes us back to the main subject:
"Whomever have said "There is no god but Allah" and has the weight of a mustard seed of faith in his heart, will leave Hellfire" (Sahih Bukhary)
The hadiths that echo the one above are in the DOZENS in both Sahih and other authenticated books of Sunnah. The established belief is that all muslims who recited the Shahada truthfully will eventually go to Paradise Allah willing. Some muslims will go to heaven without account or reckoning. Some will go to paradise after reckoning of their sins and being forgiven. Some will reside in Hell for a while until the intercession of the prophet -s.a.a.w.- relieves them out of Hellfire or the mercy of Allah catches up to them.
"[FONT=Verdana,Arial]Allah does not forgive that partners should be set up with Him; but He forgives anything else, to whom He pleases; to set up partners with Allah is indeed to devise a sin Most heinous.[/FONT]" [4:48]
We do not go further than this and start pointing fingers at who will make or who wouldn't, as this is the realm of Allah's mercy and judgement.
For those who are confused like brother Nabiel about the fairness in this, you need to learn about three things:
1- 3- Allah's name is "Al-AAdl" meaning The Just. We understand your thoughts brother Nabiel that you had, however you need to guard yourself against such thoughts and statements and learn that objections of this type are objections against Allah's rulings and a challenge to his justice, which is clearly blasphemy. Before anyone can appoint themselves as a critic of what is and is not fair, remember also that you yourself as a human have sins and would hope and pray that they are forgiven.
2- Ask yourself that: You are right to think that someone who commited crimes should be punished for them, but why would you think it is also fair that a disbeliever who rejected Allah altogether should be equated with a person who believed and accepted Allah? The verses in the Quran that describe those who go to paradise are ALWAYS starting with: "Those who believe and hold their prayers...". Any budhist or hindu like you are suggesting who does good deeds, does it for their own reasons and not for Allah and therefore have no right to ask Allah's reward specially after receiving the Quran and the messages and rejecting them. They may ask for their paradise from whatever it is they were worshipping. A muslim, no matter how bad, is still a believer in Allah, and that is the fundamental portion of our religion before being kind to others or doing the good deeds.
3- What makes you think that spending thousands (if not millions or longer) of years in hellfire suffering constant pains and torture worse than death every single moment of every hour of every day of those long thousands of years, is going to be a breeze? Would you dare three days? A lifetime equivelent to this one? Two lifetimes? Would you actually stand being next to them hearing their screaming? Additionally, when they leave Hellfire, they will enter the lower levels of paradise and reside there, while the muslims who went directly will be in the higher levels and will be rewarded as per their strength of faith and their efforts and deeds.
May Allah guard us against even the sight of Hell.