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.
Reproduced in full. Translation: Ante-Nicene Fathers, vol. 1 (Roberts and Donaldson, 1885), public domain (Public domain).