Leviticus 25:44-46

⁴⁴As for your male and female slaves, whom you can obtain from the nations who live around you, you may buy slaves from them.

⁴⁵You may also buy slaves from the foreigners who are living among you, that is, from their families who are with you, children who have been born in your land. They may become your property.

⁴⁶You may provide such slaves as an inheritance for your children after you, to hold as property, and make them slaves for life, but you must not rule over your brothers among the people of Israel with harshness.