Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 48
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 48, with English translation by Maimonides (Rambam).
להוסיף קרבן ביום הצום, שנאמר: "וּבֶעָשׂוֹר לַחֹדֶשׁ הַשְּׁבִיעִי..." (במדבר כט, ז).
To offer an additional sacrifice on the fast [of Yom Kippur], as [Numbers 29:7] states: "On the tenth of the seventh month...."