Parashas Bo · Shemos 12:4 · TT note 9
Torah Temimah on Shemos 12:4, note 9 (on אִישׁ לְפִי אׇכְלוֹ תָּכֹסּוּ) points to Pesachim 61a.
Sefaria — Exodus 12:4
Sefaria — Torah Temimah on Exodus 12:4
Sefaria — Pesachim 61a
E-Daf — Pesachim 61a
English: One might think that if it was slaughtered not for those enrolled, he has merely violated the mitzvah but it is still valid — therefore it says “according to the number … you shall make your count,” the verse repeating the matter to make it indispensable.
Repetition of בְּמִכְסַת / תָּכֹסּוּ establishes that slaughter only for enrolled participants is מעכב.
The Pesach is valid only when slaughtered for those enrolled who can eat of it; otherwise it is disqualified.