Sahih Muslim
The Book on Government (Kitab Al-Imara)
Book Number: 020 / Hadith Number: 4584
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.