Positive commandment · Mitzvat Aseh #48 of 613
To offer an additional sacrifice on the fast [of Yom Kippur]
להוסיף קרבן ביום הצום
The verse · Bamidbar 29:7
וּבֶעָשׂוֹר לַחֹדֶשׁ הַשְּׁבִיעִי הַזֶּה מִקְרָא־קֹדֶשׁ יִהְיֶה לָכֶם וְעִנִּיתֶם אֶת־נַפְשֹׁתֵיכֶם כׇּל־מְלָאכָה לֹא תַעֲשׂוּ
“And you shall have on the tenth day of this seventh month a holy gathering; and you shall afflict your souls: you shall not do any work:”
The Rambam’s words
To offer an additional sacrifice on the fast [of Yom Kippur], as [Numbers 29:7] states: "On the tenth of the seventh month...."
להוסיף קרבן ביום הצום, שנאמר: "וּבֶעָשׂוֹר לַחֹדֶשׁ הַשְּׁבִיעִי..." (במדבר כט, ז).
Why · Sefer HaChinuch #314
Additional sacrifice of Yom Kippur
“From the roots of the commandment is like the matter that we wrote above regarding the sacrifice (Sefer HaChinukh 95) — that a man is acted upon according to his deeds.”
Where in the Mishneh Torah
Hilchot Daily Offerings and Additional Offerings הלכות תמידים ומוספין