Chullin 96 - How Much of the Sciatic Sinew?
One who removes the sciatic sinew from the thigh meat must remove it entirely. He must dig deep into the thigh and extract it from wherever it is embedded in the meat. This is the presumed opinion of Rabbi Meir, because anonymous ruling are usually his. Rabbi Yehudah says that he needs only remove enough for the mitzvah, leveling it off the surface of the rounded flesh. Both agree that the Torah required only the removal of the external part of the sinew, but Rabbi Meir opines that the Sages extended this prohibition to within the thigh, and Rabbi Yehudah says that they did not enact any additional prohibitions.
Bar Piyuli was excising a sciatic sinew from a piece of thigh meat, doing this in front of Shmuel, and he was cutting it flush at the surface. Shmuel said, "Cut deeper! Had I not seen you, you would have fed me something forbidden." Bar Piyuli became frighntened, and the knife fell from his hand. "Do not be afraid," - said Shmuel, - "for you did nothing wrong. The one who taught you did it according to Rabbi Yehudah, but the law follows Rabbi Meir."
Art: Pietro Longhi - Vendor of Roast Meat