Tertullian

Further: since in the self-same commentary of Luke the third hour is demonstrated as an hour of prayer, about which hour it was that they who had received the initiatory gift of the Holy Spirit were held for drunkards; and the sixth, at which Peter went up on the roof; and the ninth, at which they entered the temple: why should we not understand that, with absolutely perfect indifference, we must pray always, and everywhere, and at every time; yet still that these three hours, as being more marked in things human-(hours) which divide the day, which distinguish businesses, which re-echo in the public ear-have likewise ever been of special solemnity in divine prayers? A persuasion which is sanctioned also by the corroborative fact of Daniel praying thrice in the day; of course, through exception of certain stated hours, no other, moreover, than the more marked and subsequently apostolic (hours)-the third, the sixth, the ninth.