Musnad Ahmad 32:1378

Hashim narrated to us, Shu'bah narrated to us, from Qatadah, who said: I heard Anas bin Malik saying, regarding the Prophet, peace and blessings of Allah be upon him, to the Ansar: "Is there anyone among you who is not of you?" They said: "A son of a sister of ours." He said: "The son of a people's sister is of them, or from among themselves."