From Sahih Muslim (Hadith 4809):
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 were 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 a 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
Reference: Muslim 4809