Positive commandment · Mitzvat Aseh #49 of 613
To carry out the service of the fast [of Yom Kippur]
לעשות עבודת היום ביום הצום
The verse · Vayikra 16:3
בְּזֹאת יָבֹא אַהֲרֹן אֶל־הַקֹּדֶשׁ בְּפַר בֶּן־בָּקָר לְחַטָּאת וְאַיִל לְעֹלָה
“Thus shall Aharon come into the holy place: with a young bullock for a sin offering, and a ram for a burnt offering.”
The Rambam’s words
To carry out the service of the fast [of Yom Kippur], as [Leviticus 16:3] states: "In this manner, Aaron will enter the [inner] sanctuary, with a young bull...." All the [particulars of] this service are stated in the parashah of Acharei Mot.
לעשות עבודת היום ביום הצום, שנאמר: "בְּזֹאת יָבֹא אַהֲרֹן אֶל הַקֹּדֶשׁ בְּפַר בֶּן בָּקָר" (ויקרא טז, ג), וכל העבודה הכתובה בפרשת "אחרי מות".
Why · Sefer HaChinuch #185
Service of Yom Kippur
“It is from the roots of the commandment that it was from the kindnesses of G-d towards His creatures to fix one day in the year for the atonement of sins, with the repentance that they repent.”
Who & when
“And all of this commandment is practiced at the time of the [Temple].”
Where in the Mishneh Torah
Hilchot Service on the Day of Atonement הלכות עבודת יום הכפורים