Parashas Bo · Shemos 12:2 · TT note 6
Chabad.org — English translation + Rashi
Torah Temimah on Shemos 12:2, note 6 (on לָכֶם) points to Rosh Hashanah 22a.
Sefaria — Exodus 12:2
Sefaria — Torah Temimah on Exodus 12:2
Sefaria — Rosh Hashanah 22a
E-Daf — Rosh Hashanah 22a
English: Rabbi Levi said: What is Rabbi Shimon’s reason? As it is written: “…This month shall be for you” — this testimony shall be valid through you. And the Rabbis? This testimony shall be handed over to you [the court].
Derash on לָכֶם: the acceptance of testimony of the new moon is entrusted to the authorized court, not left to private individuals alone.
Unlike Shabbos, which each person counts, the new month’s sanctification depends on testimony received by beis din.