Parashas Bo · Shemos 12:2 · TT note 2
Chabad.org — English translation + Rashi
Torah Temimah on Shemos 12:2, note 2 (on הַחֹדֶשׁ הַזֶּה) points to Berachos 10b.
Sefaria — Exodus 12:2
Sefaria — Torah Temimah on Exodus 12:2
Sefaria — Berachos 10b
E-Daf — Berachos 10b
English: And did Chizkiyahu not know the verse “This month shall be for you the beginning of the months”: this is Nisan, and no other is Nisan? (Teaching that one may not intercalate Nisan once it has been designated as Nisan.)
Derash on הַחֹדֶשׁ הַזֶּה fixing the identity of Nisan so that it cannot be turned into another month by intercalation after the fact.
“This month” means this is Nisan and no other month may be treated as Nisan. Once the month is sanctified as Nisan, it is not intercalated as a second Adar.