Sayyiduna Abu Juhayfah (may Allah be pleased with him) said: A woman came to the Messenger of Allah (peace and blessings be upon him), and with her was a black slave girl. The woman said: O Messenger of Allah, I am obliged to free a believing slave; will this slave girl suffice for me? The Messenger of Allah (peace and blessings be upon him) asked the slave girl: "Where is Allah?" She said: In the heaven. He (peace and blessings be upon him) said: "Who am I?" She said: You are the Messenger of Allah. He (peace and blessings be upon him) said: "Do you bear witness that there is no true god except Allah and that I am the Messenger of Allah?" She said: Yes. He (peace and blessings be upon him) said: "Do you believe in what has come from Allah?" She said: Yes. He (peace and blessings be upon him) said to the woman: "Free her, for she is a believer."
Hadith Referenceالمعجم الكبير للطبراني / باب الواو / 18517