It was narrated that Jabir Sair:"A slave came and gave his pledge to the Messenger of Allah to emigrate, and the Prophet did not realize that he was a slave. Then his master came looking for him. The Prophet said; 'Sell him to me.' So he bought him for two black slaves, then he did not accept until he had asked; 'Is he a slave?

English translation · Sunan an-Nasa'i