Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 235
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 235, with English translation by Maimonides (Rambam).
לעבוד בעבד כנעני לעולם, שנאמר: "לעולם, בהם תעבודו" (ויקרא כה,מו).
To have a Canaanite servant serve forever, as [Leviticus 25:46] states: "You shall have them serve you forever."