The 164th mitzvah is that we are commanded to fast on the tenth of Tishrei [i.e., Yom Kippur].
The source of this commandment is G‑d's statement1 (exalted be He), "You must afflict your lives."
The Sifra explains: "The expression 'You must afflict your lives' refers to 'affliction' that affects one's actual life. What is that? Eating and drinking."
The Oral Tradition2 explains that one must also refrain from bathing, anointing, wearing leather shoes, and engaging in marital relations.
The source that one must refrain from all these activities is the verse,3 "It is a Sabbath of Sabbaths to you, and you must afflict your lives." The verse says, "Sabbath of Sabbaths," to indicate that one must refrain [observe a "Sabbath"] from the various categories of melachah4 and prohibited activities, and that one must refrain [observe a "Sabbath"] from those things which nourish and sustain the body. The Sifra says, "What is the source that bathing, anointing, and marital relations are forbidden on Yom Kippur? From the verse, 'Sabbath of Sabbaths.' " This means that one must refrain [observe a "Sabbath"] from these activities in order to reach the state of affliction.5
