37 Farther, although the apostle bids us to “pray without ceasing,” and although to the saints their very sleep is a supplication, we ought to have fixed hours of prayer, that if we are detained by work, the time may remind us of our duty. Prayers, as every one knows, ought to be said at the third, sixth and ninth hours, at dawn and at evening. No meal should be begun without prayer, and before leaving table thanks should be returned to the Creator. We should rise two or three times in the night, and go over the parts of Scripture which we know by heart.
When we leave the roof which shelters us, prayer should be our armor; and when we return from the street we should pray before we sit down, and not give the frail body rest until the soul is fed. In every act we do, in every step we take, let our hand trace the Lord's cross. Speak against nobody, and do not slander your mother's son. “Who are you that judgest the servant of another? To his own lord he stands or falls; yea, he shall be made to stand, for the Lord has power to make him stand.” If you have fasted two or three days, do not think yourself better than others who do not fast.
You fast and are angry; another eats and wears a smiling face. You work off your irritation and hunger in quarrels. He uses food in moderation and gives God thanks. Daily Isaiah cries: “Is it such a fast that I have chosen, says the Lord?” and again: “In the day of your fast ye find your own pleasure, and oppress all your laborers. Behold ye fast for strife and contention, and to smite with the fist of wickedness. How fast ye unto me?” What kind of fast can his be whose wrath is such that not only does the night go down upon it, but that even the moon's changes leave it unchanged?
Source: Letters (New Advent)