Sahih al-Bukhari 3777
Narrated `Aisha:
The day of Bu'ath (i.e. Day of fighting between the two tribes of the Ansar, the Aus and Khazraj) was 
brought about by Allah for the good of His Apostle so that when Allah's Messenger (ﷺ) reached (Medina), 
the tribes of Medina had already divided and their chiefs had been killed and wounded. So Allah had 
brought about the battle for the good of H is Apostle in order that they (i.e. the Ansar) might embrace 
Islam.
حَدَّثَنِي عُبَيْدُ بْنُ إِسْمَاعِيلَ، حَدَّثَنَا أَبُو أُسَامَةَ، عَنْ هِشَامٍ، عَنْ أَبِيهِ، عَنْ عَائِشَةَ ـ رضى الله عنها ـ قَالَتْ كَانَ يَوْمُ بُعَاثَ يَوْمًا قَدَّمَهُ اللَّهُ لِرَسُولِهِ صلى الله عليه وسلم فَقَدِمَ رَسُولُ اللَّهِ صلى الله عليه وسلم وَقَدِ افْتَرَقَ مَلَؤُهُمْ، وَقُتِلَتْ سَرَوَاتُهُمْ، وَجُرِّحُوا، فَقَدَّمَهُ اللَّهُ لِرَسُولِهِ صلى الله عليه وسلم فِي دُخُولِهِمْ فِي الإِسْلاَمِ‏.‏