(18) باب اسْتِحْبَابِ مُبَايَعَةِ الإِمَامِ الْجَيْشَ عِنْدَ إِرَادَةِ الْقِتَالِ وَبَيَانِ بَيْعَةِ الرِّضْوَانِ تَحْتَ الشَّجَرَةِ
It has been narrated on the authority of Jabir who said:
If we had been a hundred thousand in number, it (the water) would have sufficed us, but actually we were fifteen hundred.
Reference: Sahih Muslim 1856 g
In-book reference: Book 33, Hadith 110
USC-MSA web (English) reference(deprecated numbering scheme): Book 20, Hadith 4582