Sahih Bukhari Volume 001, Book 002, Hadith Number 020.
Narated By Anas : The Prophet said, "Whoever possesses the following three qualities will taste the sweetness of faith:
1. The one to whom Allah and His Apostle become dearer than anything else.
2. Who loves a person and he loves him only for Allah's sake.
3. Who hates to revert to disbelief (Atheism) after Allah has brought (saved) him out from it, as he hates to be thrown in fire."