34 And in discharging the duties of prayer, we find that the three children with Daniel, being strong in faith and victorious in captivity, observed the third, sixth, and ninth hour, as it were, for a sacrament of the Trinity, which in the last times had to be manifested. For both the first hour in its progress to the third shows forth the consummated number of the Trinity, and also the fourth proceeding to the sixth declares another Trinity; and when from the seventh the ninth is completed, the perfect Trinity is numbered every three hours, which spaces of hours the worshippers of God in time past having spiritually decided on, made use of for determined and lawful times for prayer.
And subsequently the thing was manifested, that these things were of old Sacraments, in that anciently righteous men prayed in this manner. For upon the disciples at the third hour the Holy Spirit descended, who fulfilled the grace of the Lord's promise. Moreover, at the sixth hour, Peter, going up unto the house-top, was instructed as well by the sign as by the word of God admonishing him to receive all to the grace of salvation, whereas he was previously doubtful of the receiving of the Gentiles to baptism. And from the sixth hour to the ninth, the Lord, being crucified, washed away our sins by His blood; and that He might redeem and quicken us, He then accomplished His victory by His passion.
Source: The Treatises of Cyprian (New Advent)