Musnad Abu Ya'la al-Mawsili 142:3

Abu Khaythama narrated to us, Waki' narrated to us, Shu'bah narrated to us, he said: I said to Mu'awiyah ibn Qurrah, "Did you hear Anas say that the Prophet, peace and blessings of Allah be upon him, said: 'The son of a people's sister is one of them'?" He said, "Yes."