First Council of Nicaea, 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.

Translation: Henry R. Percival, The Seven Ecumenical Councils, NPNF II.14 (1900), via CCEL.