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 XII

Canon XII.

If any one, under pretence of asceticism, should wear a peribolæum and, as if this gave him righteousness, shall despise those who with piety wear the berus and use other common and customary dress, let him be anathema.

Notes.

Ancient Epitome of Canon XII.

Whoso despises those who wear beruses, let him be anathema.

Hefele.

The βήροι (lacernæ) were the common upper garments worn by men over the tunic; but the περιβόλαια were rough mantles worn by philosophers to show their contempt for all luxury.  Socrates (H. E., ii. 43) and the Synodal Letter of Gangra in its third article say that Eustathius of Sebaste wore the philosopher’s mantle.  But this canon in no way absolutely rejects a special dress for monks, for it is not the distinctive dress but the proud and superstitious over-estimation of its worth which the Synod here blames.

This canon is found in the Corpus Juris Canonici, Gratian’s Decretum, Pars I., Dist. xxx., c. xv.