Leviticus 25:45-46

45Also you may buy slaves from the children of the foreigners who reside with you, and from their families that are with you, whom they have fathered in your land, they may become your property.

46You may give them as inheritance to your children after you to possess as property. You may enslave them perpetually. However, as for your brothers the Israelites, no man may rule over his brother harshly.