Parashas Bo · Shemos 12:4 · TT note 5
Torah Temimah on Shemos 12:4, note 5 (on בְּמִכְסַת נְפָשֹׁת) points to Pesachim 81a.
Sefaria — Exodus 12:4
Sefaria — Torah Temimah on Exodus 12:4
Sefaria — Pesachim 81a
E-Daf — Pesachim 81a
English: The Sages taught: “According to the number” — this teaches that the Pesach is slaughtered only for those enrolled on it. One might think that if it was slaughtered [without that], after the fact it is valid — therefore it says “according to the number you shall make your count,” repeating the matter to make it indispensable.
Repetition of בְּמִכְסַת / תָּכֹסּוּ establishes an indispensable requirement of prior enrollment.
Slaughter of the Pesach is valid only for those already numbered upon it.