Sayyiduna al-Sharid bin Suwayd al-Thaqafi (may Allah be pleased with him) says: I said: O Messenger of Allah (peace and blessings be upon him)! My mother left instructions to free a neck (slave) on her behalf, and I have a black slave girl. He (peace and blessings be upon him) said: "Call her." He (peace and blessings be upon him) asked: "Who is your Lord?" The slave girl said: Allah. Then he (peace and blessings be upon him) asked: "Who am I?" She said: The Messenger of Allah. He (peace and blessings be upon him) said: "Free her; she is a believer."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الظهار / 15272