سنن ابن ماجه

It was narrated that Jabir said: “A slave came and gave his pledge to the Prophet (ﷺ), pledging to emigrate, and the Prophet (ﷺ) did not realize that he was a slave. Then his master came looking for him, and the Prophet (ﷺ) said: ‘Sell him to me,’ and he brought him in exchange for two black slaves. Then after that he did not accept the pledge from anyone until he had asked whether he was a slave.”