Abdullah bin Utbah narrates from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that a man came to the Prophet (peace and blessings be upon him) with a black slave girl and said: "O Messenger of Allah (peace and blessings be upon him)! I owe the freeing of a believing slave." He (peace and blessings be upon him) asked her: "Where is Allah?" She pointed with her finger to the sky. He (peace and blessings be upon him) asked: "Who am I?" She pointed to the Messenger of Allah (peace and blessings be upon him) and to the sky, meaning: You are the Messenger of Allah. He (peace and blessings be upon him) said: "She is a believer; free her."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الظهار / 15268