Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 45
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 45, with English translation by Maimonides (Rambam).
להוסיף קרבן ביום עצרת, שנאמר: "וּבְיוֹם הַבִּכּוּרִים..." (במדבר כח, כו) "וְהִקְרַבְתֶּם עוֹלָה לְרֵיחַ..." (במדבר כח, כז).
To offer an additional sacrifice on Shavuot, as [Numbers 28:26] states: "On the day of the first fruits [Bikkurim]...."