(18) باب اسْتِحْبَابِ مُبَايَعَةِ الإِمَامِ الْجَيْشَ عِنْدَ إِرَادَةِ الْقِتَالِ وَبَيَانِ بَيْعَةِ الرِّضْوَانِ تَحْتَ الشَّجَرَةِ
It has been narrated on the authority of 'Abdullah b. Abu Aufa who said:
The Companions of the Tree (i e. those who swore fealty under the tree) were one thousand and three hundred, and the people of Aslam tribe were one-eighth of the Muhajirs.
Reference: Sahih Muslim 1857 a
In-book reference: Book 33, Hadith 112
USC-MSA web (English) reference(deprecated numbering scheme): Book 20, Hadith 4584