Second Council of Nicaea, Canon XVI

That it does not become one in holy orders to be clad in costly apparel.

All buffoonery and decking of the body ill becomes the priestly rank.  Therefore those bishops and clerics who array themselves in gay and showy clothing ought to correct themselves, and if they do not amend they ought to be subjected to punishment.  So likewise they who anoint themselves with perfumes.  When the root of bitterness sprang up, there was poured into the Catholic Church the pollution of the heresy of the traducers of the Christians.  And such as were defiled by it, not only detested the pictured images, but also set at naught all decorum, being exceedingly mad against those who lived gravely and religiously; so that in them was fulfilled that which is written, “The service of God is abominable to the sinner.”  If therefore, any are found deriding those who are clad in poor and grave raiment, let them be corrected by punishment.  For from early times every man in holy orders wore modest and grave clothing; and verily whatever is worn, not so much because of necessity, as for the sake of outward show, savours of dandyism, as says Basil the Great.  Nor did anyone array himself in raiment embroidered with silk, nor put many coloured ornaments on the border of his garments; for they had heard from the lips of God that “They that wear soft clothing are in kings’ houses.”

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