Leviticus 25:42-46

This is because the Israelites are my servants, and I brought them out of slavery in Egypt. They must not become slaves again. You must not rule this person cruelly, but you must respect your God. “ ‘Your men and women slaves must come from other nations around you; from them you may buy slaves. Also you may buy as slaves children from the families of foreigners living in your land. These child slaves will belong to you, and you may even pass them on to your children after you die; you can make them slaves forever. But you must not rule cruelly over your own people, the Israelites.