Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 258
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 258, with English translation by Maimonides (Rambam).
שלא למכור אותו ממכרת עבד, שנאמר "לֹא יִמָּכְרוּ מִמְכֶּרֶת עָבֶד" (ויקרא כה, מב).
Not to sell [a Hebrew servant] as slaves are sold, as [Leviticus 25:42] states: "Do not sell him as slaves are sold."