Talmud - Pesahim 99a פסחים צ״ט א׳

From here the Sages stated: Silence is fitting for the wise, and a fortiori the same is true for fools. In the case under discussion, had neither side appointed the other to slaughter the Paschal lamb on its behalf, both offerings would be valid and would be consumed. When each side appointed the other to slaughter the Paschal lamb on its behalf, only the first one slaughtered may be eaten while the second one must be burned. This is as it is stated: “Even a fool, when he holds his peace, is considered wise; and he that shuts his lips is esteemed as a man of understanding” (Proverbs 17:28).

The mishna addressed the question of what two individuals whose Paschal lambs were intermingled should do. The Gemara suggests: Let us say that the mishna is not in accordance with the opinion of Rabbi Yehuda, as it was taught in a baraita that the verse which states: “And if the household be too little for a lamb” (Exodus 12:4) teaches that the members of the group may keep decreasing, meaning it is permissible for them to withdraw from the offering, provided one of the original members of the group remains; this is the statement of Rabbi Yehuda. Rabbi Yosei says: It is permissible for them to withdraw provided they do not leave the Paschal lamb for any amount of time on its own, without anyone registered for it. In the mishna, the two original owners forego their shares in their own Paschal lambs, and the only remaining members of each group are people who were added from the marketplace, who were not original owners from the time the animal was separated as a Paschal lamb. This is apparently not in accordance with the opinion of Rabbi Yehuda.

Rabbi Yoḥanan said: Even if you say that the mishna is in accordance with the opinion of Rabbi Yehuda, since Rabbi Yehuda said elsewhere that one may not slaughter the Paschal lamb for an individual, meaning that there must be at least two people registered for each Paschal lamb, in this case, in which only one person was registered for a Paschal lamb, from the beginning it stood to have another person registered with the original owner. Therefore, the person who joins later is like one of the original members of the group.

Rav Ashi said: The mishna is also precisely worded in accordance with the opinion of Rabbi Yehuda, as it teaches: And similarly, five groups of five each. This indicates that if each group has five, yes, the solution of sending four of them to join the other groups is viable, but if there were four groups of five and a fifth group of only four, there is no way to employ the solution of the mishna. Is it not because none of the original members of the group would remain with it if all four members of the group would join the other four groups, and it is prohibited to leave a Paschal lamb without any of the original members of its group? The Gemara concludes: Indeed, learn from it that the mishna follows the opinion of Rabbi Yehuda.

Traduction : The William Davidson digital edition of the Koren Noé Talmud, with commentary by Rabbi Adin Even-Israel Steinsaltz.

Pesachim 99a explores mixed-up Paschal lambs, withdrawal rules for offering groups, and Rabbi Yehuda's view on registered participants.