Musnad Ahmad 32:1989

Bahz narrated to us, Shu'bah narrated to us, he said: Qatadah said, he informed me from Anas ibn Malik, that the Messenger of Allah (peace and blessings of Allah be upon him) gathered the Ansar and said: "Is there anyone among you who is not of you?" They said: "No, except a son of a sister of ours." The Messenger of Allah (peace and blessings of Allah be upon him) said: "The son of a people's sister is of them." And he said once: "From them." Shu'bah said: So I mentioned that to Mu'awiyah ibn Qurrah, and he narrated it to me from Anas.