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.
حَدَّثَنِي عُبَيْدُ بْنُ إِسْمَاعِيلَ، حَدَّثَنَا أَبُو أُسَامَةَ، عَنْ هِشَامٍ، عَنْ أَبِيهِ، عَنْ عَائِشَةَ ـ رضى الله عنها ـ قَالَتْ كَانَ يَوْمُ بُعَاثٍ يَوْمًا قَدَّمَهُ اللَّهُ لِرَسُولِهِ صلى الله عليه وسلم، فَقَدِمَ رَسُولُ اللَّهِ صلى الله عليه وسلم وَقَدِ افْتَرَقَ مَلَؤُهُمْ، وَقُتِّلَتْ سَرَوَاتُهُمْ وَجُرِّحُوا، قَدَّمَهُ اللَّهُ لِرَسُولِهِ صلى الله عليه وسلم فِي دُخُولِهِمْ فِي الإِسْلاَمِ.
Bukhari
Merits of the Helpers in Madinah (Ansaar) • Hadith 71