From Sahih Muslim (Hadith 4812):
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 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
Reference: Muslim 4812