Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 42
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 42, with English translation by Maimonides (Rambam).
להוסיף קרבן בכל ראש חודש וחודש, שנאמר: "וּבְרָאשֵׁי חָדְשֵׁיכֶם תַּקְרִיבוּ..." (במדבר כח, יא).
To offer an additional sacrifice every Rosh Chodesh, as [Numbers 28:11] states: "On your Rashei Chodashim...."