Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 196
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 196, with English translation by Maimonides (Rambam).
שלא לאכול ביום הצום, שנאמר "כי כל הנפש אשר לא תעונה" (ויקרא כג,כט).
Not to eat on the day of the fast [of Yom Kippur], as [Leviticus 23:29] states: "For any person who does not afflict himself [on that day]...."