Musnad Ahmad 32:1882

'Affan narrated to us, Hammad narrated to us, he said: Thabit informed us, from Anas, that the Messenger of Allah (peace and blessings of Allah be upon him) visited a man from the Ansar and said: "O maternal uncle, say: There is no god but Allah." He said: "A maternal uncle or a paternal uncle?" He said: "Rather, a maternal uncle." He said: "And is it better for me to say it?" He said: "Yes."