Volume 1, Book 2, Number 48 :sahih bakhari

Narrated by 'Abdullah bin 'Abbas
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."