Sirach 23:9Ben Sira, Ecclesiasticus
7 Listen, my children, to instruction concerning the mouth; the one who observes it will never be caught. 8 Sinners are overtaken through their lips; by them the reviler and the arrogant are tripped up. 9 Do not accustom your mouth to oaths, nor habitually utter the name of the Holy One; 10 for as a servant who is constantly under scrutiny will not lack bruises, so also the person who always swears and utters the Name will never be cleansed from sin. 11 The one who swears many oaths is full of iniquity, and the scourge will not leave his house. If he swears in error, his sin remains on him, and if he disregards it, he sins doubly; if he swears a false oath, he will not be justified, for his house will be filled with calamities.
John Chrysostom Homily 10 on Acts 1:9
Well, then, let us speak to you today from the Old Testament. What does it tell us? Accustom not your mouth to swearing; neither use yourself to the naming of the Holy One. And why? For as a servant that is continually beaten shall not be without a blue mark, so he that swears. See the discernment of this wise man. He did not say, Accustom not to swearing your mind, but your mouth; because being altogether an affair of the mouth, thus it is easily remedied. For at last it becomes a habit without intention; as for instance, there are many who entering the public baths, as soon as they have passed the threshold, cross Hom. in themselves (σφραγίζονται). This the hand has got to do, without any one's bidding, by force of habit. Again, at the lighting of a candle, often when the mind is intent on something else, the hand makes the sign. In the same way also the mouth, without concurrence of the mind, articulates the word, from mere habit, and the whole affair is in the tongue.
Notes and References
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