Sahih Muslim Book 20, Hadith Number 4578.

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 Abu Zubair who heard Jabir being questioned as to how many people were there on the Day of Hudaibiya He replied: We wore fourteen hundred. We swore fealty to him, and Umar was holding his hand while he was sitting Under the tree (to administer the oath). The tree was Samura (a wild tree found in deserts). All of as took the oath of fealty at his hands except Jadd b. Qais al-Ansari who hid himself under the belly of his camel.