Sahih Bukhari Volume 005, Book 058, Hadith Number 121.
Hadith Section/Book: Sahih Bukhari Sahih Bukhari Book 58. Merits of the Helpers in Madina (Ansaar)
Narated By '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 Apostle 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.