Sahih Muslim Book 20, Hadith Number 4582.

Chapter : It is good on the part of a leader to take the oath of allegiance from his forces when intending to fight, and an account of the allegiance of God’s pleasure under the tree.

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.