Musannaf Ibn Abi Shayba 36:572

Yahya ibn Abi Bukayr narrated to us, Shu'bah narrated to us, from 'Amr ibn Murrah, he said: I heard Ibn Abi Awfa saying: The companions of the Prophet, may Allah bless him and grant him peace, who pledged allegiance under the tree were one thousand and four hundred or one thousand and three hundred. And Aslam were from the Emigrants (Muhajirun).