Kapłańska 16:29 Halacha: Mishneh Torah, Rest, Mishneh Torah, Positive Mitzvot, Mishneh Torah, Negative Mitzvot & Shulchan Arukh, Orach Chayim

Another positive command concerning Yom Kippur requires us to abstain from eating or drinking on that day, as it is written: "You shall afflict yourselves" (Leviticus 16:29). According to tradition, self-affliction signifies fasting. Whoever fasts on Yom Kippur fulfills a positive command, and whoever eats or drinks on it breaks a positive command and transgresses a negative command, as it is written: "Whoever will not abstain and fast on that day shall be cut off" (Leviticus 23:29). Since the verse [explicitly] punished one who did not afflict himself [with] the punishment of excision, we have [also] learned that we are warned about eating and drinking on it. And anyone who eats or drinks on it inadvertently must bring a fixed sin-offering.

To fast on the tenth day of that month, as it is said, “On the tenth day of the month, ye shall afflict your souls” (Lev. 16:27).

Not to do work on the Day of Atonement, as it is said concerning it, “Ye shall do no manner of work” (Lev. 23:31).