Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 261
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 261, with English translation by Maimonides (Rambam).
שלא למכור אמה עברייה לאחר, שנאמר "לֹא יִמְשׁוֹל לְמָכְרָהּ בְּבִגְדוֹ בָהּ" (שמות כא, ח).
Not to sell a Hebrew maidservant to another person, as [Exodus 21:8] states: "He shall have no power to sell her."