Parashas Bo · Shemos 12:5 · TT note 3
Torah Temimah on Shemos 12:5, note 3 (on בֶּן־שָׁנָה) points to Mechilta / R’ Yishmael.
Sefaria — Exodus 12:5
Sefaria — Torah Temimah on Exodus 12:5
Sefaria — Mechilta
English: “A year old” — I only have “a year old”; from where [do we learn] its entire year? Rabbi Yishmael used to say: a kal va-chomer — if an olah, which is stricter, is valid throughout its year as if it were a year old, then a Pesach, which is lighter, all the more so.
Kal va-chomer from olah to Pesach: age is counted from the animal’s own birth, for its full year.
A year-old Pesach is measured from its birth date through its complete year.