Sahih Muslim Book 20, Hadith Number 4583.

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 (through a different chain of transmitters) on the authority of Salim b. al-Ja’d who said: I asked Jabir: How many were you on the Day of Hudaibiya? He said: One thousand and four hundred.