Nicene and Post-Nicene Fathers, Ser. II, Vol. XIV:
The Canons of the Councils of Ancyra, Gangra, Neocæsarea, Antioch and Laodicea, which Canons were Accepted and Received by the Ecumenical Synods.: Canon XVIII

p. 134 Canon XVIII.

The same service of prayers is to be said always both at nones and at vespers.

Notes.

Ancient Epitome of Canon XVIII.

The same prayers shall be said at nones and vespers.

Hefele.

Some feasts ended at the ninth hour, others only in the evening, and both alike with prayer.  The Synod here wills that in both cases the same prayers should be used.  Thus does Van Espen explain the words of the text, and I think rightly.  But the Greek commentator understands the Synod to order that the same prayers should be used in all places, thus excluding all individual caprice.  According to this, the rule of conformity would refer to places; while, according to Van Espen, the nones and vespers were to be the same.  If, however, this interpretation were correct, the Synod would not have only spoken of the prayers at nones and vespers, but would have said in general, “all dioceses shall use the same form of prayer.”