Parashas Bo · Shemos 12:2 · TT note 5
Chabad.org — English translation + Rashi
Torah Temimah on Shemos 12:2, note 5 points to Menachos 29a (Rosh Chodesh was difficult for Moshe until shown with the finger).
Sefaria — Exodus 12:2
Sefaria — Torah Temimah on Exodus 12:2
Sefaria — Menachos 29a
E-Daf — Menachos 29a
English: A beraisa of the school of Rabbi Yishmael taught: Three things were difficult for Moshe until the Holy One, blessed be He, showed them to him with His finger: the Menorah, Rosh Chodesh, and the sheratzim. … Rosh Chodesh — as it is written: “This month shall be for you the beginning of the months.”
The word הַזֶּה (“this”) is read as pointing: God indicated the new moon to Moshe by demonstration.
Sanctifying the new month was not self-evident; Moshe needed divine demonstration of what constitutes the visible new moon.