Positive commandment · Mitzvat Aseh #164 of 613
To fast on the tenth [of this month, Yom Kippur,]
להתענות בעשירי בו
The verse · Vayikra 16:29
וְהָיְתָה לָכֶם לְחֻקַּת עוֹלָם בַּחֹדֶשׁ הַשְּׁבִיעִי בֶּעָשׂוֹר לַחֹדֶשׁ תְּעַנּוּ אֶת־נַפְשֹׁתֵיכֶם וְכׇל־מְלָאכָה לֹא תַעֲשׂוּ הָאֶזְרָח וְהַגֵּר הַגָּר בְּתוֹכְכֶם
“And this shall be a statute for ever to you: that in the seventh month, on the tenth day of the month, you shall afflict your souls, and do no work at all, the home born or the stranger that sojourns among you:”
The Rambam’s words
To fast on the tenth [of this month, Yom Kippur,] as [Leviticus 16:29] states: "On the tenth of the month, you shall afflict your souls."
להתענות בעשירי בו, שנאמר: "ובעשור לחודש תענו את נפשותיכם" (ראה ויקרא טז,כט; במדבר כט,ז).
Why · Sefer HaChinuch #313
Fast on the tenth day of Tishrei
“It is from the roots of the commandment that it was from the kindnesses of G-d towards His creatures to establish one day to atone for their sins with repentance — and as I wrote at length in the Order of Acharei Mot (Sefer HaChinukh 185) in the commandment of the service for Yom Kippur.”
Who & when
“And [it] is practiced in every place and at all times by males and females.”
Where in the Mishneh Torah
Hilchot Rest on the Tenth of Tishrei הלכות שביתת עשור