Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 167
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 167, with English translation by Maimonides (Rambam).
לשבות בשמיני של חג, שנאמר: "וביום השמיני מקרא קודש" (ראה ויקרא כג,לו).
To rest on the eighth day of the festival of Sukkot, as [Leviticus 23:36] states: "On the eighth day, there shall be a sacred holiday."