Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 153
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 153, with English translation by Maimonides (Rambam).
לקדש חודשים ולחשב שנים וחודשים בבית דין בלבד, שנאמר: "החודש הזה לכם, ראש חודשים" (שמות יב,ב).
To sanctify the months and to calculate the years and months. [This mitzvah is incumbent on] the court alone, as [Exodus 12:2] states: "This month will be for you the first of the months."