Sahih Bukhari Volume 4, Book 56, Hadith 2805

حَدَّثَنَا يَحْيَى بْنُ بُكَيْرٍ، حَدَّثَنَا اللَّيْثُ، قَالَ حَدَّثَنِي يُونُسُ، عَنِ ابْنِ شِهَابٍ، عَنْ عُبَيْدِ اللَّهِ بْنِ عَبْدِ اللَّهِ، أَنَّ عَبْدَ اللَّهِ بْنَ عَبَّاسٍ، أَخْبَرَهُ أَنَّ أَبَا سُفْيَانَ أَخْبَرَهُ أَنَّ هِرَقْلَ قَالَ لَهُ سَأَلْتُكَ كَيْفَ كَانَ قِتَالُكُمْ إِيَّاهُ فَزَعَمْتَ أَنَّ الْحَرْبَ سِجَالٌ وَدُوَلٌ، فَكَذَلِكَ الرُّسُلُ تُبْتَلَى ثُمَّ تَكُونُ لَهُمُ الْعَاقِبَةُ

Narrated By Abdullah bin Abbas : That Abu Sufyan told him that Heraclius said to him, "I asked you about the outcome of your battles with him (i.e. the Prophet) and you told me that you fought each other with alternate success. So the Apostles are tested in this way but the ultimate victory is always theirs.