Leviticus 25:53

[And] as a yearly hired servant shall he be with him
Being redeemable every year, and upon his redemption might quit his master's service, as an hireling may; and the price of his redemption to be valued according to the years he served, and as if he had been hired for so much a year; as well as he was to be treated in a kind and gentle manner, not as a bondman, but as if he was an hired servant, as follows: [and the other] shall not rule with rigour over him in thy sight;
the person he is sold unto, his master, a sojourner or stranger, he might not use an Hebrew he had bought with any severity; for if an Hebrew master might not use an Hebrew servant with rigour, it was not by any means to be admitted in the commonwealth of Israel for a proselyte to use one in such a manner, and that openly, in the sight of an Israelite his neighbour; he looking on and not remonstrating against it, or acquainting the civil magistrate with it, who had it in his power to redress such a grievance, and ought to do it.