The Canons of the Council in Trullo 27

27 Canon XXVII

None of those who are in the catalogue of the clergy shall wear clothes unsuited to them, either while still living in town or when on a journey: but they shall wear such clothes as are assigned to those who belong to the clergy. And if any one shall violate this canon, he shall be cut off for one week.

Ancient Epitome of Canon XXVII.

A clergyman must not wear an unsuitable dress either when travelling or when at home. Should he do so, he shall be cut off for one week.

Philip Schaff and Henry Wace (eds.), New York: Christian Literature Publishing Co., 1890-1900. Public domain: English translation first published 1885-1900 (Nicene and Post-Nicene Fathers, Second Series).