(27) باب الْقَسَامَةُ فِي الْجَاهِلِيَّةِ
Narrated 'Aisha:
Allah caused the day of Buath to take place before Allah's Messenger (ﷺ) was sent (as an Apostle) so that when Allah's Messenger (ﷺ) reached Medina, those people had already divided (in different groups) and their chiefs had been killed or wounded. So Allah made that day precede Allah's Messenger (ﷺ) so that they (i.e. the Ansar) might embrace Islam.
Reference: Sahih al-Bukhari 3846
In-book reference: Book 63, Hadith 72
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 5, Book 58, Hadith 186