Musnad Ahmad 32:834

Bahz narrated to us, Shu'bah narrated to us, Qatadah narrated to us, he informed me from Anas ibn Malik, that the Messenger of Allah (peace and blessings of Allah be upon him), when he gathered the Ansar, said: "Is there anyone among you who is not of you?" They said: "No, except for 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 one of them" — or he said: "is of the people." He said: So I mentioned that to Mu'awiyah ibn Qurrah, and he narrated to me from Anas.