Musnad Ahmad 32:247

Waki’ narrated to us, Shu’bah narrated to us, he said: I said to Mu’awiyah ibn Qurrah: Did you hear Anas say: The Messenger of Allah, peace and blessings of Allah be upon him, said to al-Nu’man ibn Muqarrin: "The son of a people's sister is one of them"? He said: Yes.