הֶֽחָכָם֙ עֵינָ֣יו בְּרֹאשׁ֔וֹ וְהַכְּסִ֖יל בַּחֹ֣שֶׁךְ הוֹלֵ֑ךְ וְיָדַ֣עְתִּי גַם־אָ֔נִי שֶׁמִּקְרֶ֥ה אֶחָ֖ד יִקְרֶ֥ה אֶת־כֻּלָּֽם׃
דער חכם האָט זײַנע אױגן אין קאָפּ, און דער נאַר גײט אין דער פֿינצטער.אָבער איך האָב אױך געמערקט אַז אײן טראַף טרעפֿט זײ אַלעמען.
Shulchan Arukh, Orach Chayim
The Laws Pertaining to the Three [Shabbos] Meals, 6 Seifim:
One should be very cautious to have a third meal, and even if he is satisfied he can fulfill it with about the volume of an egg. And if he cannot eat at all, he need not cause himself discomfort. And a wise person will not fill his stomach with the morning meal, so as to save room for a third meal. Rema: And one who did not eat on Shabbat evening should eat three meals on the day of Shabbat. (the Rosh)
One should be very cautious to have a third meal, and even if he is satisfied he can fulfill it with about the volume of an egg. And if he cannot eat at all, he need not cause himself discomfort. And a wise person will not fill his stomach with the morning meal, so as to save room for a third meal. Rema: And one who did not eat on Shabbat evening should eat three meals on the day of Shabbat. (the Rosh)