Chullin, Chapter 7, Mishnah 6. This chapter is devoted to the prohibition of gid hanasheh, the sciatic nerve that may not be eaten from an animal. Our mishnah asks a basic question about the reach of that prohibition: does it attach to every animal, or only to the animals we are permitted to eat in the first place?
The Ruling of the Mishnah
The mishnah teaches: "Noheg batehorah ve'eino noheg bateme'ah" - the law of gid hanasheh operates in a kosher animal and does not operate in a non-kosher animal. The Torah's prohibition was stated with regard to the animals that are otherwise fit for eating, and there alone it takes hold.
Where does this matter in practice? In the tally of lashes. A person who eats the sciatic nerve of an animal from a permitted species has violated the prohibition of gid hanasheh and receives malkut for it. A person who eats that same nerve from a forbidden species has plainly done something wrong as well: the flesh he swallowed came from an animal he may not eat, and lashes are administered for that. But the mishnah gives him no additional malkut beyond that, since within a forbidden species the law of gid hanasheh never took hold to begin with.
Rabbi Yehuda's Position
"Rabbi Yehuda omer: Af bateme'ah" - Rabbi Yehuda maintains that the prohibition operates in a forbidden species too. On his view, a person who consumed such a nerve is liable to malkut twice over: once because the flesh of that animal was forbidden to him, and once more because a sciatic nerve is forbidden to him.
The Proof from the Sons of Yaakov
Rabbi Yehuda supplies the logic behind his position: "Vahalo mibnei Yaakov nasar gid hanasheh" - the sons of Yaakov Avinu were already keeping away from the sciatic nerve, "ve'adayin behemah teme'ah" - and at that stage animals of forbidden species were "muteret lahen", still available to them for eating.
The background is the well known episode: Yaakov wrestled with the angel and was struck on the hip, and as a lasting remembrance of that struggle we refrain from eating the gid hanasheh. Rabbi Yehuda reads that as the moment the mitzvah was given, in the generation of the Avot. Now, in those days there was no prohibition against eating a non-kosher animal; every animal was permitted to them. If the sons of Yaakov were nevertheless commanded not to eat the gid hanasheh, the prohibition must have covered every animal they might eat, including what we call non-kosher. That is Rabbi Yehuda's svara, and from it he concludes that the law still extends to a non-kosher animal today.
The Answer of the Sages
The tanna kamma, the anonymous Sages of our mishnah, answer him ("amru lo") by denying his premise outright: "B'Sinai ne'emar" - this command was spoken at Sinai - "ela shenichtav bimkomo" - and Scripture simply set it down in its own place.
In other words, the sons of Yaakov were never in fact bound by this prohibition. The commandment of gid hanasheh was first given at Har Sinai together with the rest of the mitzvot. The Torah simply recorded it in the place where it belongs thematically, within the account of Yaakov's struggle with the angel, since that is the event it commemorates. If so, the narrative in Bereishit cannot serve as evidence about the era of the Avot, and Rabbi Yehuda's proof falls away. The prohibition was given at Sinai to a people already forbidden to eat non-kosher animals, and it therefore applies only to a kosher animal.