Zevachim 23A

לפי אכלו״! למצוה.

according to his eating, you should make your count for the lamb” (Exodus 12:4)? This teaches that one may bring the Paschal offering only if he is able to partake of it. The Talmud responds: This requirement is also meant as a mitzva ab initio; it does not disqualify the offering if not fulfilled.

ולעכובי לא?! והתניא: ״במכסת נפשות״⁠ – מלמד שאין הפסח נשחט אלא למנוייו. שחטו שלא למנוייו ⁠– יכול יהא כעובר על המצוה? תלמוד לומר: ״תכסו״⁠ – הכתוב שנה עליו לעכב. ואיתקש אוכלין למנויין!

The Talmud asks: And is this requirement not indispensable even after the fact? But isn’t it taught in a baraita that the phrase “according to the number of the souls” teaches that the Paschal offering may be slaughtered only for those who registered for it in advance. If the Paschal offering was slaughtered for individuals who did not register for it, one might have thought that it would only be like transgressing a mitzva, but the offering would not be disqualified. Therefore, the verse states: “You should make your count”; the verse repeats the issue of counting to stress that the halakha is indispensable, and if one slaughters the offering for one who is not registered, it is disqualified. The Talmud concludes: And those who eat the offering are juxtaposed to those registered for it, as the verse states: “According to the number of the souls; a man, according to his eating.” Accordingly, if one slaughters the Paschal offering for one who cannot partake of it, the offering is disqualified.

זקני דרום לא מקשי. וכי לא מקשי נמי ⁠– מהא נמי אית להו פירכא: ומה במקום שנטמאו בעלים בשרץ, שמשלחין קרבנותיהם לכתחילה ⁠– כהן שנטמא בשרץ אינו מרצה; מקום שנטמאו בעלים במת, שאין משלחין קרבנותיהן לכתחילה ⁠– כהן שנטמא במת אינו דין שאינו מרצה?!

The Talmud responds: The Elders of the South do not juxtapose the phrases, i.e., they do not interpret the verse’s juxtaposition of the two phrases as significant. The Talmud asks: But even if they do not juxtapose the phrases, there is a refutation to their statement from this inference as well: And just as in a case where the owner became impure due to a creeping animal, where he may send his offerings for sacrifice ab initio, a priest who became impure due to a creeping animal nevertheless cannot effect acceptance, then in a case where the owner became impure due to a corpse, where he may not send his offerings ab initio, is it not right that a priest who became impure due to a corpse cannot effect acceptance?