Musar על עבודה זרה 36:14

And let one learn from what we see of the prisoners who are brought before the judge for cross-examination. They guard their mouths and do not utter even a single word which might incriminate them. And they also take upon themselves all of the blows and revilings in order not to admit the truth, even though they know that in either event they will not live forever, and that even if they do confess, it is very likely that they will not be executed, only a minority of a minority meeting such a fate. In spite of this, they would rather take all of these sufferings upon themselves because of the possibility of a possibility of thereby lengthening their temporal lives. How much, then, should those people restrain themselves [(from speaking what is forbidden)], who sit in the company of mockers a few hours, without anyone beating or forcing them thereto. And if they restrain themselves from speaking what is forbidden, they merit eternal life, as we find in the Midrash, that for every moment that a man muzzles his mouth he merits the secreted light, which no angel or creation can conceptualize; and, conversely, they are punished for every article of speech. As the GRA has written in his holy letter, Alim Litrufah "(Koheleth 6:7): 'All of a man's toil is for his mouth.' Our sages of blessed memory have said (Chullin 89a) that all of a man's mitzvoth and Toroth do not suffice [to offset] what he utters with his mouth. 'What is the [proper] profession of a man in this world? Let him make himself mute and clamp his lips like two millstones, etc.' And all kaf hakela ["hollow of the sling" punishments] come because of the breath of the mouth of idle talk. For each article of [such] speech, he must be slung from one end of the world to the other. And all this, with superfluous speech. But with forbidden speech, such as lashon hara, leitzanuth, oaths, vows, quarrels, and curses, especially in the synagogue and on Sabbaths and festivals — for these one must descend to the nethermost depths of Sheol. And it is impossible to conceptualize the greatness of the suffering and afflictions that one must undergo for one article of [forbidden] speech. And there is not even one such article which is not written down. Angels accompany each man constantly and do not separate from him and write down each and every article of speech (Koheleth 10:20): 'For a bird of heaven will carry the voice and the winged one will tell the word.'" And we have learned (Avodah Zarah 18b): R. Katina said: "All who engage in levity fall into Gehinnom."