And the 47th mitzvah is that we are commanded to bring an offering in addition [to the daily and musaf Rosh Chodesh offerings1] on the first day of Tishrei. This is the musaf Rosh HaShanah offering.

The source of this commandment is G‑d's statement2 (exalted be He), "On the first day of the seventh month...you must present a burnt-offering as an appealing fragrance to G‑d."