Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 259
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 259, with English translation by Maimonides (Rambam).
שלא לעבוד בעבד עברי בפרך, שנאמר "לֹא תִרְדֶּה בּוֹ בְּפָרֶךְ" (ויקרא כה, מג).
Not to make a Hebrew servant perform rigorous work, as [Leviticus 25:43] states: "Do not rule over him with rigor."