VI. No one but priests are allowed to preach.
This too it behooves you, beloved, to guard against, that no one except those who are the Lord's priests dare to claim the right of teaching or preaching, be he monk or layman, who boasts himself of some knowledge. Because although it is desirable that all the Church's sons should understand the things which are right and sound, yet it is permitted to none outside the priestly rank to assume the office of preacher, since in the Church of God all things ought to be orderly, that in Christ's one body the more excellent members should fulfil their own duties, and the lower not resist the higher. Dated the 11th of June, in the consulship of the illustrious Opilio (453).
Source: Letters (New Advent)