(18) باب اسْتِحْبَابِ مُبَايَعَةِ الإِمَامِ الْجَيْشَ عِنْدَ إِرَادَةِ الْقِتَالِ وَبَيَانِ بَيْعَةِ الرِّضْوَانِ تَحْتَ الشَّجَرَةِ
The above tradition has also been handed down through a different chain of transmitters.
Reference: Sahih Muslim 1860 b
In-book reference: Book 33, Hadith 119
USC-MSA web (English) reference(deprecated numbering scheme): Book 20, Hadith 4591