Musnad Ahmad 584:1

Abd al-Samad narrated to us, Hammad ibn Salamah narrated to us, Muhammad ibn Amr narrated to us, from Abu Salamah, from al-Sharid: That his mother had bequeathed that a believing slave be emancipated on her behalf. So he asked the Messenger of Allah (peace and blessings of Allah be upon him) about that, and he said: "I have a black or Nubian slave girl; shall I emancipate her?" He said: "Bring her." So I called her, and she came. He said to her: "Who is your Lord?" She said: "Allah." He said: "Who am I?" She said: "You are the Messenger of Allah." He said: "Emancipate her, for she is a believer."