Leviticus 25:44-46

As for your male and female slaves whom you may have—you may acquire male and female slaves from the pagan nations that are around you. Then, too, it is out of the sons of the sojourners who live as aliens among you that you may gain acquisition, and out of their families who are with you, whom they will have produced in your land; they also may become your possession. You may even bequeath them to your sons after you, to receive as a possession; you can use them as permanent slaves. But in respect to your countrymen, the sons of Israel, you shall not rule with severity over one another.