Pesachim 73A

״עבודת מתנה אתן את כהנתכם והזר הקרב יומת״, עשו אכילת תרומה בגבולין כעבודת בית המקדש.

“I have given you the priesthood as a service of gift; and the stranger that comes near should be put to death” (Numbers 18:7). This verse is found in the context of the priestly gifts, including teruma, and comes to teach us that they made the eating of teruma in the outlying areas, i.e., outside the Temple, like the service of the Temple.

שחטו שלא לאוכליו. פשיטא! כיון דהתם ⁠— פסול, הכא ⁠— חייב! משום דתנא סיפא פטור, תנא רישא חייב.

We learned in our Mishnah that if one unwittingly slaughtered the Paschal lamb on Shabbat for those who cannot eat it or for those who did not register for it, he is liable to bring a sin-offering. The Talmud asks: It is obvious. Since there, with regard to the slaughter itself, it is invalid, here, with regard to Shabbat, he is liable, for it turns out that he performed a prohibited labor that was not necessary for sacrificing an offering. The Talmud answers: Since the latter clause of the Mishnah taught cases in which he is exempt from bringing a sin-offering, the first clause taught cases in which he is liable, even though it does not really teach us anything new.

והא נמי פשיטא, משום דהתם כשר, הכא פטור! אלא, איידי דתנא שחטו שלא לשמו בשבת, תנא נמי שלא לאוכליו. והיא גופא למה לי? משום דקבעי לאיפלוגי רבי אליעזר ורבי יהושע.

The Talmud asks: But this also is obvious, since there, the offering is valid, here with regard to Shabbat he is exempt, as the slaughter did not involve a desecration of Shabbat. Rather, since the Mishnah taught the case of one who slaughtered the Paschal lamb for a different purpose on Shabbat, it also taught the case of one who slaughtered it for those who cannot eat it. The Talmud asks further: And it itself, the case of slaughtering the Paschal offering for a different purpose, why do I need it? The halakha there is also obvious. The Talmud answers: Since the Mishnah wished to teach the dispute between R' Eliezer and R' Yehoshua, it taught all these other halakhot as well.