Nicene and Post-Nicene Fathers, Ser. II, Vol. XIV:
The First Ecumenical Council:  The First Council of Nice.: Canon XX

Canon XX.

Forasmuch as there are certain persons who kneel on the Lord’s Day and in the days of Pentecost, therefore, to the intent that all things may be uniformly observed everywhere (in every parish), it seems good to the holy Synod that prayer be made to God standing.

Notes.

Ancient Epitome of Canon XX.

On Lord’s days and at Pentecost all must pray standing and not kneeling.

Hammond.

Although kneeling was the common posture for prayer in the primitive Church, yet the custom had prevailed, even from the earliest times, of standing at prayer on the Lord’s day, and during the fifty days between Easter and Pentecost.  Tertullian, in a passage in his treatise De Corona Militis, which is often quoted, mentions it amongst other observances which, though not expressly commanded in Scripture, yet were universally practised upon the authority of tradition.  “We consider it unlawful,” he says, “to fast, or to pray kneeling, upon the Lord’s day; we enjoy the same liberty from Easter-day to that of Pentecost.”  De Cor. Mil. s. 3, 4.  Many other of the Fathers notice the same practice, the reason of which, as given by Augustine and others, was to commemorate the resurrection of our Lord, and to signify the rest and joy of our own resurrection, which that of our Lord assured.  This canon, as Beveridge observes, is a proof of the importance formerly attached to an uniformity of sacred rites throughout the Church, which made the Nicene Fathers thus sanction and enforce by their authority a practice which in itself is indifferent, and not commanded directly or indirectly in Scripture, and assign this as their reason for doing so:  “In order that all things may be observed in like manner in every parish” or diocese.

Hefele.

All the churches did not, however, adopt this practice; for we see in the Acts of the Apostles (Acts 20:36, Acts 21:5) that St. Paul prayed kneeling during the time between Pentecost and Easter.

This canon is found in the Corpus Juris Canonici.  Decretum, Pars III, De Conc. Dist. III. c. x.