Mukhtasar Muslim

Jabir (may Allah be pleased with him) reports that a slave came and pledged allegiance to the Messenger of Allah (peace and blessings be upon him) for migration. But the Prophet (peace and blessings be upon him) did not know that he was a slave. Then his owner came to take him back, so the Messenger of Allah (peace and blessings be upon him) said, "Sell him to me." Then the Prophet (peace and blessings be upon him) bought him by giving two black slaves. After that, the Prophet (peace and blessings be upon him) would not take allegiance from anyone until he asked whether he was a slave or free.