Y'la bin Muslim bin Hrmz يعلى بن مسلم بن هرمز · The Book of al-Bay'ah

"I pledged to the Messenger of Allah to be sincere toward ever Muslim."

"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?"'