Responsa zu Tehillim 32:8

They responded: "Let me enlighten you and show you the way," (Psalms 32:8) for this matter can be learned from the Torah of our teacher Moses. For it is written (Exodus 16:25) "Eat it today, for today is a sabbath of God; you will not find it today on the plain." Just as the days are separated from each other¹i.e, the three mentions of the word 'today' in the verse, so too do the times of the three meals need to be separated from each other.