Sahih Muslim Book 020, Hadith Number 4581.
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 Salim b. Abu al-Ja'd who said: I asked Jabir b. 'Abdullah about the number of the Companions (of the Holy Prophet who took the oath of fealty under) the tree. He said: If we were a hundred thousand, it (i.e. the water in the well at Hudaibiya) would have sufficed us, but actually we were one thousand and five hundred.