Summary

Talmud (Rodkinson Edition) — Book 3…

If the original was found before the sacrifice had been slaughtered and had been exchanged for another animal at the same time; but if it was found before and was exchanged after that, the substitute may be offered up as a peace-offering. Why is this so? Because the slaughtering sanctifies the animal which is substituted at the time when it may still be killed; but an animal which is exchanged after the slaughter, not being suitable for a paschal sacrifice, cannot be slaughtered.
Source: Wikisource

Talmud (Rodkinson Edition) — Book 3…

Now, according to R. Jehudah, who states that if a man can reach the court of the Temple in time for eating, the paschal sacrifice may be slaughtered for him, why does he hold that the paschal offering must not be slaughtered for one who became unclean through a reptile? A man in such a condition becomes clean and may enter the Temple after sunset, and at that time the pashal lamb is eaten.
Source: Wikisource

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