Parashas Bo · Shemos 12:2 · TT note 9
Chabad.org — English translation + Rashi
Torah Temimah on Shemos 12:2, note 9 (on רֹאשׁ חֳדָשִׁים) points to Mechilta.
Sefaria — Exodus 12:2
Sefaria — Torah Temimah on Exodus 12:2
Sefaria — Mechilta
English: “Beginning of the months” — I might hear only a minimum of two months; from where [do we include] the other months? Therefore it says: “of the months of the year.”
Ribbui from לְחׇדְשֵׁי הַשָּׁנָה: the phrase extends the counting beyond a mere minimum of months.
Nisan is the head of the full set of months of the year, not merely of a limited pair of months.