Leviticus 25:46; Leviticus 25:53

46 '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.

53 'Like a man hired year by year he shall be with him; he shall not rule over him with severity in your sight.