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 Messenger so that when Allah’s Messenger 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 His Messenger in order that they (i.e. the Ansar) might embrace Islam.”

Sahih Bukhari