It is narrated from Sayyiduna Jabir (may Allah be pleased with him) that a slave came and pledged allegiance to the Prophet (peace and blessings be upon him) for migration. The Prophet (peace and blessings be upon him) did not know that he was a slave. Meanwhile, his master came searching for him. The Prophet (peace and blessings be upon him) said: "Sell him to me." And he 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 not.