Abba Poemen the Great ca. 4th cent.
Teach your mouth to say that which you have in your heart. (Sayings of the Desert Fathers: Poemen 63)
St. Benedict of Nursia ca. 480-547
Let us therefore consider how we ought to conduct ourselves in sight of the Godhead and of His Angels, and let us take part in the psalmody in such a way that our mind may be in harmony with our voice. (The Rule of St. Benedict, 19)