Tertullian, On Prayer Chapter 25: Of Time for Prayer

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. The first infusion of the Holy Spirit into the congregated disciples took place at “the third hour.” Peter, on the day on which he experienced the vision of Universal Community, (exhibited) in that small vessel, had ascended into the more lofty parts of the house, for prayer’s sake “at the sixth hour.” The same (apostle) was going into the temple, with John, “at the ninth hour,” when he restored the paralytic to his health. Albeit these practices stand simply without any precept for their observance, still it may be granted a good thing to establish some definite presumption, which may both add stringency to the admonition to pray, and may, as it were by a law, tear us out from our businesses unto such a duty; 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.