Sa'id bin Jubayr سعيد بن جبير الأسدي · The Book of al-Bay'ah

"A slave came and pledged to the Prophet 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,' and he bought him for two black slaves. Then he did not accept the pledge from anyone until he asked: 'Is he a slave?"'