Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 260
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 260, with English translation by Maimonides (Rambam).
שלא להניח הגוי לעבד בעבד עברי הנמכר לו בפרך, שנאמר "לֹא יִרְדֶּנּוּ בְּפָרֶךְ לְעֵינֶךָ" (ויקרא כה, נג).
Not to allow a gentile [who purchased a Hebrew servant] to make him work rigorously, as [Leviticus 25:53] states: "He shall not rule over him with rigor."