Masechet Chullin, Perek Zayin, Mishnah Gimel. The Mishnah says, “Ha'ochel mi'gid ha'nasheh (sciatic nerve) kezayit (olive-sized amount) sofeg et ha'arba'im.” It's a kezayit word. Of a gid ha'nasheh, they received 40 lashes. They violated a Torah prohibition by eating a kezayit. Therefore, they would receive malkot (lashes), 40.
“Achalo ve'en bo kezayit chayav,” if he ate all of the gid ha'nasheh. But it was a small gid ha'nasheh, and it didn't even equal up to a kezayit. We say he's still liable for malkot since he ate an entire beriyah, an entire creation. Usually, when the Torah forbids a person to eat something, he has to eat a kezayit in order to violate it and receive malkot. But if he eats an entire creation, a full beriyah, there's a different story. For example, let's say he ate a full bug in its entirety, and that bug was smaller than a kezayit; he would still receive malkot. “Achal mizeh kezayit u'mizeh kezayit sofeg shmonim,” if he ate a kezayit from one thigh and a kezayit from the other thigh of the animal, he receives 80 lashes. There are two separate penalties of malkot. He ate the gid ha'nasheh of two different thighs, right and left. They're both forbidden, and therefore he would receive 80 lashes.
Rabbi Yehuda says, “No, he wouldn't receive 80. Rather, he would only receive 40, because the gid ha'nasheh of the right thigh is the only one that's forbidden.” He learns that from the Pasuk (verse), where it says that the gid ha'nasheh is not allowed to be eaten. But then it says, “which is on the spoon of the thigh,” which is saying it's only one of them. If it's only one of them, we would assume that it's the right one, the one that's on the right side of the animal—that's the only one that would be forbidden. So that's how Rabbi Yehuda argues with the Tanna Kamma (the first Tanna).