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."
حَدَّثَنَا إِبْرَاهِيمُ بْنُ حَمْزَةَ، قَالَ حَدَّثَنَا إِبْرَاهِيمُ بْنُ سَعْدٍ، عَنْ صَالِحٍ، عَنِ ابْنِ شِهَابٍ، عَنْ عُبَيْدِ اللَّهِ بْنِ عَبْدِ اللَّهِ، أَنَّ عَبْدَ اللَّهِ بْنَ عَبَّاسٍ، أَخْبَرَهُ قَالَ أَخْبَرَنِي أَبُو سُفْيَانَ، أَنَّ هِرَقْلَ، قَالَ لَهُ سَأَلْتُكَ هَلْ يَزِيدُونَ أَمْ يَنْقُصُونَ، فَزَعَمْتَ أَنَّهُمْ يَزِيدُونَ، وَكَذَلِكَ الإِيمَانُ حَتَّى يَتِمَّ‏.‏ وَسَأَلْتُكَ هَلْ يَرْتَدُّ أَحَدٌ سَخْطَةً لِدِينِهِ بَعْدَ أَنْ يَدْخُلَ فِيهِ، فَزَعَمْتَ أَنْ لاَ، وَكَذَلِكَ الإِيمَانُ حِينَ تُخَالِطُ بَشَاشَتُهُ الْقُلُوبَ، لاَ يَسْخَطُهُ أَحَدٌ‏.‏