Positive commandment · Mitzvat Aseh #42 of 613
To offer an additional sacrifice every Rosh Chodesh
להוסיף קרבן בכל ראש חודש וחודש
The verse · Bamidbar 28:11
וּבְרָאשֵׁי חׇדְשֵׁיכֶם תַּקְרִיבוּ עֹלָה לַה׳ פָּרִים בְּנֵי־בָקָר שְׁנַיִם וְאַיִל אֶחָד כְּבָשִׂים בְּנֵי־שָׁנָה שִׁבְעָה תְּמִימִם
“And in the beginnings of your months you shall offer a burnt offering to the Lord; two young bullocks, and one ram, seven lambs of the first year without blemish;”
The Rambam’s words
To offer an additional sacrifice every Rosh Chodesh, as [Numbers 28:11] states: "On your Rashei Chodashim...."
להוסיף קרבן בכל ראש חודש וחודש, שנאמר: "וּבְרָאשֵׁי חָדְשֵׁיכֶם תַּקְרִיבוּ..." (במדבר כח, יא).
Why · Sefer HaChinuch #403
Additional sacrifice on each and every month
“From the roots of the commandment, [there is a need to] preface that it is known to all wise-hearted men that the sphere of the sun and the sphere of the moon act with their power — that is given over to them from the Master of all the powers in this lowly world — great effects upon the bodies of people and upon all types of other animals.”
Where in the Mishneh Torah
Hilchot Daily Offerings and Additional Offerings הלכות תמידים ומוספין