Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 47
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 47, with English translation by Maimonides (Rambam).
להוסיף קרבן בראש השנה, שנאמר: "וּבַחֹדֶשׁ הַשְּׁבִיעִי בְּאֶחָד לַחֹדֶשׁ [...] וַעֲשִׂיתֶם..." (במדבר כט, א).
To offer an additional sacrifice on Rosh HaShanah, as [Numbers 29:1] states: "And in the seventh month, on the first of the month...."