Musnad Ahmad 33:883

Abu Sa’id, the freed slave of Banu Hashim, narrated to us, Layth narrated to us, Abu al-Zubayr narrated to us, from Jabir, who said: "A slave came to the Prophet, peace and blessings of Allah be upon him, and pledged allegiance to him. Then his master came and recognized him, so the Messenger of Allah, peace and blessings of Allah be upon him, bought him from him and set him free. Thereafter, he would not accept the pledge of allegiance from anyone until he asked him whether he was a free man or a slave."