Musnad Ahmad 33:655

Hujayn narrated to us, Layth narrated to us, from Abu al-Zubayr; and Ishaq ibn Isa narrated to us, Layth narrated to us, Abu al-Zubayr narrated to me, from Jabir ibn Abdullah, who said: A slave came and pledged allegiance to the Messenger of Allah, peace and blessings of Allah be upon him, for migration, and the Messenger of Allah, peace and blessings of Allah be upon him, did not know that he was a slave. Then his master came seeking him. The Prophet, peace and blessings of Allah be upon him, said: "Sell him to me." So he bought him for two black slaves. Thereafter, he would not accept the pledge of allegiance from anyone until he asked him: "Is he a slave?"