Job 21:5

“Ver. 5. Mark me, and be astonished. 43. i.e. Consider what I have done, and be astonished at the things that I undergo under this infliction of the rod. And he yet further rightly introduces the words; And lay your finger upon your mouth. As if he had said in plain speech; 'Knowing the good things that I have done and seeing the ills that I am subjected to, your own selves keep even from offence in words, and in my strokes dread your own hurts.' Or indeed seeing that by our fingers we distinguish things severally, discretion is not unfitly represented by the fingers; and hence it is said by the Psalmist, Blessed be the Lord my God, Which teacheth my hands to war, and my fingers to fight [Ps. 144, 1]; i.e. by the 'hands' denoting practice, and by the 'fingers,' discretion. And so the finger is laid to the mouth, when the tongue is bridled by discretion, that by what it utters, it may not fall into the sin of foolishness. And therefore he says, Lay your finger upon your mouth; i.e. 'join the virtue of discretion to your speech, that in those things which ye say light against the hypocrite, ye may see to what persons they are proper to be said.'”

“Since this disputation is about a great matter, one should listen attentively, and so he says, "Pay attention to me." It should also not be listened to lightly or with scorn, but more with dignity and with amazement, and so he says, "Be astonished." It should also be heard in silence and without murmuring, and so he says, "and put your finger over your mouth."”