Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 166
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 166, with English translation by Maimonides (Rambam).
לשבות בראשון של חג הסוכות, שנאמר: "ביום הראשון, מקרא קודש" (ויקרא כג,לה).
To rest on the first day of the festival of Sukkot, as [Leviticus 23:35] states: "On the first day, there shall be a sacred holiday."