Musnad Ahmad 32:601

'Abd al-Samad narrated to us, Hammad narrated to us, Thabit narrated to us, from Anas, that the Prophet, peace and blessings of Allah be upon him, entered upon a man from Banu al-Najjar to visit him. The Messenger of Allah, peace and blessings of Allah be upon him, said to him: "O maternal uncle, say: There is no god but Allah." He said: "Am I a maternal uncle or a paternal uncle?" The Prophet, peace and blessings of Allah be upon him, said: "No, rather a maternal uncle." He said to him: "Say: There is no god but He." He said: "Is it better for me?" He said: "Yes."