Shabbat 118A

מצילין מזון שלש סעודות, מאי לאו ⁠— דלא אכל! לא, דאכל. שחרית מצילין מזון שתי סעודות, מאי לאו ⁠— דלא אכל? לא, דאכל. במנחה מצילין מזון סעודה אחת, מאי לאו ⁠— דלא אכל? לא, דאכל!

one rescues food for 3 meals. What, is this not referring to a situation where he has not yet eaten any meals? Apparently, one is obligated to eat only 3 meals on Shabbat. The Talmud rejects this: No, it is referring to a situation where one already ate one meal. 3 meals still remain, in accordance with the opinion of R' Ḥidka, who requires 4 meals. The Talmud challenges this from the continuation of the Mishnah: If a fire ignited in the morning, one may only rescue food for two meals. What, is it not referring to a situation where one has not yet eaten one’s morning meal and the obligation is to eat only 3 meals, one at night and two during the day? The Talmud rejects this: No, it is referring to a situation where one has already eaten the morning meal. The Talmud challenges this from the continuation of the Mishnah: If a fire ignited in the afternoon, one may rescue food for one meal. What, is it not referring to a situation where one has not yet eaten the afternoon meal and the obligation is to eat only 3 meals? The Talmud rejects this: No, it is referring to a situation where one has already eaten.

והא מדקתני סיפא: רבי יוסי אומר לעולם מצילין מזון שלש סעודות ⁠— מכלל דתנא קמא שלש סבירא ליה! אלא מחוורתא, מתניתין דלא כרבי חידקא.

The Talmud challenges this: From the fact that it is taught in the latter clause of the Mishnah: R' Yosei says: One may always rescue food for 3 meals, by inference, even the first tanna holds that the obligation is to eat 3 meals, as there is no dispute with regard to the number of meals on Shabbat. Rather, it is clear that the Mishnah is not in accordance with the opinion of R' Ḥidka. According to the tanna’im in the Mishnah, one is obligated to eat 3 meals on Shabbat.