Sahih Muslim Book 20, Hadith Number 4584.

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 ‘Abdullah b. Abu Aufa who said: The Companions of the Tree (i.e. those who swore fealty under the tree) were one thousand and three hundred, and the people of Aslam tribe were one-eighth of the Muhajirs.