St. Ambrose on Speaking. Or not.

Manifold dangers are incurred by speaking; the remedy for which Scripture shows to consist in silence.

Now what ought we to learn before everything else, but to be silent, that we may be able to speak? Lest my voice should condemn me, before that of another acquit me; for it is written: By your words you shall be condemned. Matthew 12:37 What need is there, then, that you should hasten to undergo the danger of condemnation by speaking, when you can be more safe by keeping silent? How many have I seen to fall into sin by speaking, but scarcely one by keeping silent; and so it is more difficult to know how to keep silent than how to speak. I know that most persons speak because they do not know how to keep silent. It is seldom that any one is silent even when speaking profits him nothing. He is wise, then, who knows how to keep silent. Lastly, the Wisdom of God said: The Lord has given to me the tongue of learning, that I should know when it is good to speak. Justly, then, is he wise who has received of the Lord to know when he ought to speak. Wherefore the Scripture says well: A wise man will keep silence until there is opportunity. Sirach 20:7

Therefore the saints of the Lord loved to keep silence, because they knew that a man's voice is often the utterance of sin, and a man's speech is the beginning of human error. Lastly, the Saint of the Lord said: I said, I will take heed to my ways, that I offend not in my tongue. For he knew and had read that it was a mark of the divine protection for a man to be hid from the scourge of his own tongue, Job 5:21 and the witness of his own conscience. We are chastised by the silent reproaches of our thoughts, and by the judgment of conscience. We are chastised also by the lash of our own voice, when we say things whereby our soul is mortally injured, and our mind is sorely wounded. But who is there that has his heart clean from the impurities of sin, and does not offend in his tongue? And so, as he saw there was no one who could keep his mouth free from evil speaking, he laid upon himself the law of innocency by a rule of silence, with a view to avoiding by silence that fault which he could with difficulty escape in speaking.

Let us hearken, then, to the master of precaution: I said, I will take heed to my ways; that is, I said to myself: in the silent biddings of my thoughts, I have enjoined upon myself, that I should take heed to my ways. Some ways there are which we ought to follow; others as to which we ought to take heed. We must follow the ways of the Lord, and take heed to our own ways, lest they lead us into sin. One can take heed if one is not hasty in speaking. The law says: Hear, O Israel, the Lord your God. Deuteronomy 6:4 It said not: Speak, but Hear. Eve fell because she said to the man what she had not heard from the Lord her God. The first word from God says to you: Hear! If you hear, take heed to your ways; and if you have fallen, quickly amend your way. For: Wherein does a young man amend his way; except in taking heed to the word of the Lord? Be silent therefore first of all, and hearken, that you fail not in your tongue.

http://www.newadvent.org/fathers/34011.htm