Sahih Bukhari, Chapter: Belief

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