Leviticus 25:45-46

45Then, 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 1produced in your land; they also may become your possession. 46You may even bequeath them to your sons after you, to receive as a possession; you can use them as permanent slaves. 2But in respect to your 3countrymen, the sons of Israel, you shall not rule with severity over one another.