Psuedo-Justin:
Question 115. If bending the knee in prayer commends those who pray to God more than praying while standing, and more strongly draws down divine mercy, why do those who pray on the Lord’s Days and from Easter to Pentecost not bend the knee? And whence did this custom enter the churches?
Answer. Because we must always remember both things: our fall into sin, and the grace of Christ through which we have risen from sin; therefore, the bending of the knees on the six days is a sign of our fall into sin; but that on the Lord’s Day we do not bend the knee is a sign of the resurrection, through which by the grace of Christ we have been set free from sins and from the death owed for sin, which has been destroyed. This custom has its beginning from apostolic times, as blessed Irenaeus, martyr and bishop of Lyons, says in his book On Easter, in which he also mentions Pentecost, on which we do not bend the knee because it is of the same significance as the Lord’s Day, according to the reason given concerning this day. — Questions and Answers to the Orthodox, Question 115, traditionally attributed to Pseudo-Justin (PG 6.1363, 1366)
Irenaeus:
This [custom], of not bending the knee upon Sunday, is a symbol of the resurrection, through which we have been set free, by the grace of Christ, from sins, and from death, which has been put to death under Him. Now this custom took its rise from apostolic times, as the blessed Irenæus, the martyr and bishop of Lyons, declares in his treatise On Easter, in which he makes mention of Pentecost also; upon which [feast] we do not bend the knee, because it is of equal significance with the Lord’s day, for the reason already alleged concerning it. — Irenaeus, Fragments (7)
Tertullian:
“As often as the anniversary comes round, we make offerings for the dead as birthday honours. We count fasting or kneeling in worship on the Lord’s day to be unlawful. We rejoice in the same privilege also from Easter to Whitsunday.” — Tertullian, The Chaplet, chapter 3
We, however (just as we have received), only on the day of the Lord’s Resurrection ought to guard not only against kneeling, but every posture and office of solicitude; deferring even our businesses lest we give any place to the devil. Ephesians 4:27 Similarly, too, in the period of Pentecost; which period we distinguish by the same solemnity of exultation. But who would hesitate every day to prostrate himself before God, at least in the first prayer with which we enter on the daylight? At fasts, moreover, and Stations, no prayer should be made without kneeling, and the remaining customary marks of humility; for (then) we are not only praying, but deprecating, and making satisfaction to God our Lord. Touching times of prayer nothing at all has been prescribed, except clearly to pray at every time and every place.” — Tertullian, On Prayer Chapter 23
Peter of Alexandria:
No one shall find fault with us for observing the fourth day of the week, and the preparation, on which it is reasonably enjoined us to fast according to the tradition. On the fourth day, indeed, because on it the Jews took counsel for the betrayal of the Lord; and on the sixth, because on it He himself suffered for us. But the Lord’s day we celebrate as a day of joy, because on it He rose again, on which day we have received it for a custom not even to bow the knee. — Canonical Epistle, chapter 15
Apostolic Constitutions:
And on the day of our Lord’s resurrection, which is the Lord’s day, meet more diligently, sending praise to God that made the universe by Jesus, and sent Him to us, and condescended to let Him suffer, and raised Him from the dead. Otherwise what apology will he make to God who does not assemble on that day to hear the saving word concerning the resurrection, on which we pray thrice standing in memory of Him who arose in three days, in which is performed the reading of the prophets, the preaching of the Gospel, the oblation of the sacrifice, the gift of the holy food? — Apostolic Constitutions, book 2, chapter 59
Council of Nicaea:
Since there are some persons who kneel in church on Sundays and on the days of Pentecost, with a view to preserving uniformity in all parishes, it has seemed best to the holy council for prayers to be offered to God while standing. — Canon XX
Basil the Great:
Thus we all look to the East at our prayers, but few of us know that we are seeking our own old country, Paradise, which God planted in Eden in the East. Genesis 2:8 We pray standing, on the first day of the week, but we do not all know the reason. On the day of the resurrection (or standing again Grk. ἀ νάστασις) we remind ourselves of the grace given to us by standing at prayer, not only because we rose with Christ, and are bound to seek those things which are above, Colossians 3:1 but because the day seems to us to be in some sense an image of the age which we expect, wherefore, though it is the beginning of days, it is not called by Moses first, but one. For he says There was evening, and there was morning, one day, as though the same day often recurred. Now one and eighth are the same, in itself distinctly indicating that really one and eighth of which the Psalmist makes mention in certain titles of the Psalms, the state which follows after this present time, the day which knows no waning or eventide, and no successor, that age which ends not or grows old. Of necessity, then, the church teaches her own foster children to offer their prayers on that day standing, to the end that through continual reminder of the endless life we may not neglect to make provision for our removal there. Moreover all Pentecost is a reminder of the resurrection expected in the age to come. For that one and first day, if seven times multiplied by seven, completes the seven weeks of the holy Pentecost; for, beginning at the first, Pentecost ends with the same, making fifty revolutions through the like intervening days. And so it is a likeness of eternity, beginning as it does and ending, as in a circling course, at the same point. On this day the rules of the church have educated us to prefer the upright attitude of prayer, for by their plain reminder they, as it were, make our mind to dwell no longer in the present but in the future. Moreover every time we fall upon our knees and rise from off them we show by the very deed that by our sin we fell down to earth, and by the loving kindness of our Creator were called back to heaven. — On The Holy Spirit, chapter 27
Jerome:
For many other observances of the Churches, which are due to tradition, have acquired the authority of the written law, as for instance the practice of dipping the head three times in the laver, and then, after leaving the water, of tasting mingled milk and honey in representation of infancy; and, again, the practices of standing up in worship on the Lord’s day, and ceasing from fasting every Pentecost; and there are many other unwritten practices which have won their place through reason and custom. So you see we follow the practice of the Church, although it may be clear that a person was baptized before the Spirit was invoked. — Jerome, The Dialogue Against the Luciferians chapter 8
John Cassian:
But it is now time to follow out the plan of the promised discourse. So then when Abbot Theonas had come to visit us in our cell during Eastertide2185 after Evensong was over we sat for a little while on the ground and began diligently to consider why they were so very careful that no one should during the whole fifty days either bend his knees in prayer2186 or venture to fast till the ninth hour, and we made our inquiry the more earnestly because we had never seen this custom so carefully observed in the monasteries of Syria. — John Cassian, Conference 21, chapter 11 (420 AD)
Council of Trullo (692):
We have received it canonical from our God-bearing Fathers not to bend the knee on Sundays when honoring the resurrection of Christ. Since this observation may not be clear to some of us, we are making it plain to the faithful, that after the entrance of those in holy orders into the sacrificial altar on the evening of the Saturday in question, let none of them bend the knee until the evening of the following Sunday, when, following the entrance after the lamps have been lit, again bending knees, we thus begin to offer our prayers to the Lord. For, inasmuch as we have received it that the night succeeding Saturday was the precursor of our Savior’s rising, we commence our hymns at this point in a spiritual manner, ending the festival by passing out of darkness into light, in order that we may hence celebrate the resurrection together for a whole day and a whole night. — Canon XC of the Council of Trullo, held in conjunction with the Sixth Ecumenical Council