Here's Canon XX from the First Ecumenical Council of Nicaea:
"Since there are some who kneel on Sunday and during the season of Pentecost, this holy synod decrees that, so that the same observances may be maintained in every diocese, one should offer one's prayers to the Lord standing."
http://www.piar.hu/councils/ecum01.htm And another version from Fordham University:
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 ohservances
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(xx. 36 and xxi. 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 Cone. Dist. III. c. x.
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http://www.fordham.edu/halsall/basis/nicea1.txt
