Pesachim 97A

יכול אף לפני הפסח כן? תלמוד לומר: ״הוא״⁠ — הוא קרב, ואין תמורת הפסח קריבה.

One might have thought that even a substitute Paschal lamb that was found before the slaughter of the replacement Paschal lamb should have the same status, and it should be permitted to sacrifice such a lamb as a peace-offering. Therefore, the verse states: “It,” to emphasize that it, a valid Paschal lamb, is sacrificed, and the substitute of a Paschal lamb is not sacrificed.

היכי דמי? אילימא שנמצא קודם שחיטה והמיר בו קודם שחיטה ⁠— פשיטא! למה לי קרא? אלא לאו, שנמצא קודם שחיטה והמיר בו אחר שחיטה! תיובתא דרבא, תיובתא.

What are the circumstances of the case under discussion? If we say it is a case in which the lamb is found before the slaughter and one performed the substitution before the slaughter of the replacement, it is obvious; why do I need a specific verse to teach this halakha? Rather, is it not addressing a case in which the original lamb was found before the slaughter and one performed the substitution after the slaughter of the replacement, and the baraita ruled that the substitute lamb may not be sacrificed as a peace-offering, in opposition to the ruling of Rava? The Talmud concludes that the refutation of the opinion of Rava is indeed a conclusive refutation, and his opinion is rejected according to this version.

אמר שמואל: כל שבחטאת מתה ⁠— בפסח קרב שלמים, וכל שבחטאת רועה ⁠— בפסח נמי רועה. ורבי יוחנן אמר: אין הפסח קרב שלמים אלא שנמצא אחר שחיטה, אבל קודם שחיטה ⁠— לא.

Shmuel stated a principle pertaining to the halakhot of offerings: With regard to any animal that was consecrated as an offering and becomes unfit such that a sin-offering in its condition would be placed in isolation for it to die, meaning that it would be caused to die, if it is a Paschal lamb in that condition it is sacrificed as a peace-offering. And with regard to any animal that becomes unfit such that a sin-offering in its condition is left to graze until it develops a blemish, if it is a Paschal lamb it is also left to graze. And R' Yoḥanan said: A Paschal lamb is sacrificed as a peace-offering only when the lost lamb was found after the slaughter of the replacement Paschal lamb, but if it was found before the slaughter, there is no instance in which it is brought as a peace-offering.