Musnad Ahmad 32:621

Hassan ibn Musa narrated to us, Hammad ibn Salamah narrated to us, from Thabit al-Bunani, from Anas ibn Malik, 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: "Am I a maternal uncle or a paternal uncle?" He said: "No, rather a maternal uncle." He said: "Then is it better for me to say: There is no god but Allah?" The Prophet (peace and blessings of Allah be upon him) said: "Yes."