Tosefta for Leviticus 25:39

וְכִֽי־יָמ֥וּךְ אָחִ֛יךָ עִמָּ֖ךְ וְנִמְכַּר־לָ֑ךְ לֹא־תַעֲבֹ֥ד בּ֖וֹ עֲבֹ֥דַת עָֽבֶד׃

And if thy brother be waxen poor with thee, and sell himself unto thee, thou shalt not make him to serve as a bondservant.