Touching the time, however, the extrinsic observance of certain hours will not be unprofitable—those common hours, I mean, which mark the intervals of the day—the third, the sixth, the ninth—which we may find in the Scriptures to have been more solemn than the rest.
… so that—what we read to have been observed by Daniel also, in accordance (of course) with Israel’s discipline—we pray at least not less than thrice in the day, debtors as we are to Three—Father, Son, and Holy Spirit: of course, in addition to our regular prayers which are due, without any admonition, on the entrance of light and of night. But, withal, it becomes believers not to take food, and not to go to the bath, before interposing a prayer; for the refreshments and nourishments of the spirit are to be held prior to those of the flesh, and things heavenly prior to things earthly.
Tertullian
On Prayer
Chapter XXV