Musnad Abu Ya'la al-Mawsili 117:238

Zuhayr narrated to us, Affan narrated to us, Hammad ibn Salamah narrated to us, Thabit informed us, from Anas, that the Messenger of Allah (may Allah bless him and grant him peace) visited a man from the Ansar and said, "O maternal uncle, say: There is no god but Allah." He said, "Am I a maternal uncle or a paternal uncle?" He said, "Nay, a maternal uncle." He said, "Is it better for me to say it?" He said, "Yes."