Sahih Muslim
The Book on Government (Kitab Al-Imara)
Book Number: 020 / Hadith Number: 4581
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 Holy 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.