What is seemly is often found in the sacred writings long before it appears in the books of the philosophers. Pythagoras borrowed the law of his silence from David. David's rule, however, is the best, for our first duty is to have due measure in speaking.
30. We are instructed and taught that “what is seemly” is put in our Scriptures in the first place. (In Greek it is called πρέπον.) For we read: “A Hymn beseems You, O God, in Sion.” In Greek this is: Σοί πρέπει ὕμνος ὁ θεὸς ἐν Σιών. And the Apostle says: “Speak the things which become sound doctrine.” And elsewhere: “For it beseemed Him through Whom are all things and for Whom are all things, in bringing many sons unto glory, to make the Captain of their salvation perfect through sufferings.”
31. Was Panætius or Aristotle, who also wrote on duty, earlier than David? Why, Pythagoras himself, who lived before the time of Socrates, followed the prophet David's steps and gave his disciples a law of silence. He went so far as to restrain his disciples from the use of speech for five years. David, on the other hand, gave his law, not with a view to impair the gift of nature, but to teach us to take heed to the words we utter. Pythagoras again made his rule, that he might teach men to speak by not speaking. But David made his, so that by speaking we might learn the more how to speak. How can there be instruction without exercise, or advance without practice?
32. A man wishing to undergo a warlike training daily exercises himself with his weapons. As though ready for action he rehearses his part in the fight and stands forth just as if the enemy were in position before him. Or, with a view to acquiring skill and strength in throwing the javelin, he either puts his own arms to the proof, or avoids the blows of his foes, and escapes them by his watchful attention. The man that desires to navigate a ship on the sea, or to row, tries first on a river. They who wish to acquire an agreeable style of singing and a beautiful voice begin by bringing out their voice gradually by singing. And they who seek to win the crown of victory by strength of body and in a regular wrestling match, harden their limbs by daily practice in the wrestling school, foster their endurance, and accustom themselves to hard work.
33. Nature herself teaches us this in the case of infants. For they first exercise themselves in the sounds of speech and so learn to speak. Thus these sounds of speech are a kind of practice, and a school for the voice. Let those then who want to learn to take heed in speaking not refuse what is according to nature, but let them use all watchful care; just as those who are on a watchtower keep on the alert by watching, and not by going to sleep. For everything is made more perfect and strong by exercises proper and suitable to itself.
34. David, therefore, was not always silent, but only for a time; not perpetually nor to all did he refuse to speak; but he used not to answer the enemy that provoked him, the sinner that exasperated him. As he says elsewhere: “As though he were deaf he heard not them that speak vanity and imagine deceit: and as though he were dumb he opened not his mouth to them.” Again, in another place, it is said: “Answer not a fool according to his folly, lest you also be like to him.”
35. The first duty then is to have due measure in our speech. In this way a sacrifice of praise is offered up to God; thus a godly fear is shown when the sacred Scriptures are read; thus parents are honoured. I know well that many speak because they know not how to keep silence. But it is not often any one is silent when speaking does not profit him. A wise man, intending to speak, first carefully considers what he is to say, and to whom he is to say it; also where and at what time. There is therefore such a thing as due measure in keeping silence and also in speaking; there is also such a thing as a due measure in what we do. It is a glorious thing to maintain the right standard of duty.
Source: On the Duties of the Clergy (New Advent)