The same care must be taken that our speech proceed not from evil passions, but from good motives; for here it is that the devil is especially on the watch to catch us.
14. If any one takes heed to this, he will be mild, gentle, modest. For in guarding his mouth, and restraining his tongue, and in not speaking before examining, pondering, and weighing his words— as to whether this should be said, that should be answered, or whether it be a suitable time for this remark— he certainly is practising modesty, gentleness, patience. So he will not burst out into speech through displeasure or anger, nor give sign of any passion in his words, nor proclaim that the flames of lust are burning in his language, or that the incentives of wrath are present in what he says. Let him act thus for fear that his words, which ought to grace his inner life, should at the last plainly show and prove that there is some vice in his morals.
15. For then especially does the enemy lay his plans, when he sees passions engendered in us; then he supplies tinder; then he lays snares. Wherefore the prophet says not without cause, as we heard read today: “Surely He has delivered me from the snare of the hunter and from the hard word.” Symmachus said this means “the word of provocation;” others “the word that brings disquiet.” The snare of the enemy is our speech— but that itself is also just as much an enemy to us. Too often we say something that our foe takes hold of, and whereby he wounds us as though by our own sword. How far better it is to perish by the sword of others than by our own!
16. Accordingly the enemy tests our arms and clashes together his weapons. If he sees that I am disturbed, he implants the points of his darts, so as to raise a crop of quarrels. If I utter an unseemly word, he sets his snare. Then he puts before me the opportunity for revenge as a bait, so that in desiring to be revenged, I may put myself in the snare, and draw the death-knot tight for myself. If any one feels this enemy is near, he ought to give greater heed to his mouth, lest he make room for the enemy; but not many see him.
Source: On the Duties of the Clergy (New Advent)