Sahih Bukhari Volume 008, Book 073, Hadith Number 067.

Narated By Anas bin Malik : The Prophet said, "None will have the sweetness (delight) of Faith (a) till he loves a person and loves him only for Allah's sake, (b) and till it becomes dearer to him to be thrown in the fire than to revert to disbelief (Heathenism) after Allah has brought him out of it, (c) and till Allah and His Apostle become dearer to him than anything else."