Negative commandment · Lo Ta’aseh #260 of 613
To not allow a gentile to work a Hebrew slave sold to him
שלא להניח הגוי לעבד בעבד עברי הנמכר לו בפרך
The verse · Vayikra 25:53
כִּשְׂכִיר שָׁנָה בְּשָׁנָה יִהְיֶה עִמּוֹ לֹא־יִרְדֶּנּוּ בְּפֶרֶךְ לְעֵינֶיךָ
“As a yearly hired servant shall he be with him: and the other shall not rule with rigour over him in thy sight.”
The Rambam’s words
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."
שלא להניח הגוי לעבד בעבד עברי הנמכר לו בפרך, שנאמר "לֹא יִרְדֶּנּוּ בְּפָרֶךְ לְעֵינֶךָ" (ויקרא כה, נג).
Why · Sefer HaChinuch #348
To not allow a gentile to work a Hebrew slave sold to him
“The root of this commandment is revealed to all who see the sun. Its laws are included in its essence.”
Who & when
“And [it] is practiced by males and females at the time that our hand is powerful over the nations, that we have the power with us over them to command them to do a thing or not to do it.”
Where in the Mishneh Torah
Hilchot Slaves הלכות עבדים