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 there searching for him. The Prophet (peace and blessings be upon him) bought him and set him free. From that time onwards, the Prophet (peace and blessings be upon him) would not take the pledge of allegiance until he asked whether the person was a slave or free.