Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 165
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 165, with English translation by Maimonides (Rambam).
לשבות ביום הצום, שנאמר: "שבת שבתון" (ויקרא טז,לא; ויקרא כג,לב).
To rest on [this] fast day, as [Leviticus 16:31] states: "It shall be a Sabbath of Sabbaths...."