كتاب الإمارة
33  
The Book on Government
(18)
Chapter: It is recommended for the army to swear allegiance to the ruler when intending to fight, and an account of Bay'at Ar-Ridwan beneath the tree
(18)
باب اسْتِحْبَابِ مُبَايَعَةِ الإِمَامِ الْجَيْشَ عِنْدَ إِرَادَةِ الْقِتَالِ وَبَيَانِ بَيْعَةِ الرِّضْوَانِ تَحْتَ الشَّجَرَةِ
Sahih Muslim 1856 g

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.
وَحَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، وَابْنُ، نُمَيْرٍ قَالاَ حَدَّثَنَا عَبْدُ اللَّهِ بْنُ إِدْرِيسَ، ح وَحَدَّثَنَا رِفَاعَةُ بْنُ الْهَيْثَمِ، حَدَّثَنَا خَالِدٌ، - يَعْنِي الطَّحَّانَ - كِلاَهُمَا يَقُولُ عَنْ حُصَيْنٍ، عَنْ سَالِمِ بْنِ أَبِي الْجَعْدِ، عَنْ جَابِرٍ، قَالَ لَوْ كُنَّا مِائَةَ أَلْفٍ لَكَفَانَا كُنَّا خَمْسَ عَشْرَةَ مِائَةً ‏.‏
Reference : Sahih Muslim 1856g
In-book reference : Book 33, Hadith 110
USC-MSA web (English) reference : Book 20, Hadith 4582
  (deprecated numbering scheme)