Sahih Bukhari

I was informed by Abu Sufyan that Heraclius said to him, "I asked youwhether they (followers of Muhammad) were increasing or decreasing.You replied that they were increasing. And in fact, this is the way oftrue Faith till it is complete in all respects. I further asked youwhether there was anybody, who, after embracing his (the Prophets)religion (Islam) became displeased and discarded it. You replied inthe negative, and in fact, this is (a sign of) true faith. When itsdelight enters the heart and mixes with them completely, nobody can bedispleased with it."