Musnad Ahmad 30:788

Yazid narrated to us, al-Mas'udi informed us, from 'Awn, from his brother 'Ubayd Allah ibn 'Abd Allah ibn 'Utbah, from Abu Hurayrah: That a man came to the Prophet, may Allah bless him and grant him peace, with a black, non-Arab slave girl. He said: "O Messenger of Allah, I have an obligation to free a believing slave." The Messenger of Allah said to her: "Where is Allah?" She pointed with her index finger toward the sky. He said to her: "Who am I?" She pointed with her finger toward the Messenger of Allah and toward the sky, meaning: "You are the Messenger of Allah." He said: "Free her."