The Council in Trullo (Quinisext Council, 692 AD)

It does not befit a layman to dispute or teach publicly, thus claiming for himself authority to teach, but he should yield to the order appointed by the Lord, and to open his ears to those who have received the grace to teach, and be taught by them divine things; for in one Church God has made “different members,” according to the word of the Apostle: and Gregory the Theologian, wisely interpreting this passage, commends the order in vogue with them saying: “This order brethren we revere, this we guard. Let this one be the ear; that one the tongue, the hand or any other member. Let this one teach, but let that one learn.” And a little further on: “Learning in docility and abounding in cheerfulness, and ministering with alacrity, we shall not all be the tongue which is the more active member, not all of us Apostles, not all prophets, nor shall we all interpret.” And again: “Why dost thou make thyself a shepherd when thou art a sheep? Why become the head when thou art a foot? Why dost thou try to be a commander when thou art enrolled in the number of the soldiers?” And elsewhere: “Wisdom orders, Be not swift in words; nor compare thyself with the rich, being poor; nor seek to be wiser than the wise.” But if any one be found weakening the present canon, he is to be cut off for forty days.

Ancient Epitome of Canon LXIV.

A layman shall not teach, for all are not prophets, nor all apostles.

Zonaras points out that this canon refers only to public instruction and not to private. Van Espen further notes that in the West this restriction is limited to the solemn and public preaching and announcing of the Word of God, which is restricted to bishops, and only by special and express license given to the other clergy.