Musnad Ahmad 32:1792

Aswad ibn Amir narrated to us, Sharik narrated to us, from Abdullah ibn Isa, from Abdullah ibn Jabr, from Anas, who said: The Prophet, peace and blessings of Allah be upon him, visited a young boy who used to serve him, who was a Jew. He said to him: "Say: There is no god but Allah." He said: So he began to look at his father. He said: So he said to him: "Say what he says to you." He said: So he said it. Then the Messenger of Allah, peace and blessings of Allah be upon him, said to his companions: "Pray over your brother." And someone other than Aswad said: "I bear witness that there is no god but Allah, and that I am the Messenger of Allah." He said: So he said to him: "Say what Muhammad says to you."