"Sahih Muslim, Book 10, Number 3901:Narrated Jabir ibn Abdullah:There came a slave and pledged allegiance to Allah's Apostle (peace_be_upon_him) on migration; he (the Prophet) did not know that he was a slave. Then there came his master and demanded him back, whereupon Allah's Apostle (peace_be_upon_him) said: Sell him to me. And he bought him for two black slaves, and he did not afterwards take allegiance from anyone until he had asked him whether he was a slave (or a free man)."