Negative commandment · Lo Ta’aseh #261 of 613
Not to sell a Hebrew maidservant to another person
שלא למכור אמה עברייה לאחר
The verse · Shemot 21:8
אִם־רָעָה בְּעֵינֵי אֲדֹנֶיהָ אֲשֶׁר־[לוֹ] יְעָדָהּ וְהֶפְדָּהּ לְעַם נׇכְרִי לֹא־יִמְשֹׁל לְמׇכְרָהּ בְּבִגְדוֹ־בָהּ
“If she please not her master, who has designated her for himself, then shall he let her be redeemed: to sell her to a strange nation he shall have no power, seeing he has dealt deceitfully with her.”
The Rambam’s words
Not to sell a Hebrew maidservant to another person, as [Exodus 21:8] states: "He shall have no power to sell her."
שלא למכור אמה עברייה לאחר, שנאמר "לֹא יִמְשׁוֹל לְמָכְרָהּ בְּבִגְדוֹ בָהּ" (שמות כא, ח).
Why · Sefer HaChinuch #45
That the one who acquires the Hebrew bondwoman from the father not sell her
“From the roots of this commandment is that G-d wanted to give us merit and commanded us to act with the trait of compassion, which is beloved in front of Him.”
Where in the Mishneh Torah
Hilchot Slaves הלכות עבדים