On the first Shabbos after Moshe had informed the Jewish people that no Mon would fall on Shabbos, but that instead, a double portion would fall on Friday, he told the people, "Eat it today, because today is HaShem's Shabbos. You will not find anything in the field today." (Shemos 16:25). The Gemara in Shabbos (117b) derives from the fact that the Torah uses the word "hayom", today, three times in this Posuk (Ibid.) that there is an obligation to eat three meals on Shabbos. The Gemara further on (Ibid. 118b) documents the great rewards given to one who is careful to always eat these three meals.
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