Sahih al-Bukhari, 51

Narrated '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

Rapporté par 'Abdullah bin 'Abbas : Abu Sufyan m’a informé qu’Héraclius lui a dit : « Je t’ai demandé si les adeptes de Muhammad augmentaient ou diminuaient. Tu as répondu qu’ils augmentaient. En fait, c’est ainsi que la vraie foi progresse jusqu’à être complète. Je t’ai aussi demandé s’il y avait quelqu’un qui, après avoir embrassé sa religion (l’islam), s’en détournait et la rejetait. Tu as répondu non, et en effet, c’est un signe de la vraie foi : quand sa douceur entre dans le cœur et s’y mélange complètement, personne ne peut la rejeter. »