Shemirat HaLashon שמירת הלשון

One must take extreme care to habituate himself to heed the exhortation of Chazal (Kethuvoth 5a): "The Rabbis taught: 'One must never allow his ears to hear idle talk, for they are 'burnt' first of all the organs.'" That is, just as with a natural fire, if it enters a house, the place that it enters first is first to be burnt, so with the potent spiritual fire, the fire of Gehinnom, which is created by the tumah ["uncleanliness"] by which a man defiles his ears, that organ which is first to be defiled is readied to be seized upon immediately by the heavenly punishment, the fire of Gehinnom. Therefore, as it concerns forbidden speech, the ears are the fist organs involved in the issur, and, consequently, are defiled immediately. And through them the [forbidden] speech enters afterwards into the heart and defiles it (if he accepts it and believes it) by the issur of lashon hara. For he transgresses thereby "You shall not bear a false report," which is "heart-centered." (And so, likewise, with other things that are contrary to the Torah.) This, as we find in Sefer Charedim, in respect to negative commandments which are contingent upon the heart. For this reason, the ears are "readied" first to receive the fire of Gehinnom. And this certainly applies to hearing lashon hara against one's friend, where the sin is exceptionally great.

Therefore, if one recognizes men who are scoffers or speakers of lashon hara, he must be especially careful not to enter their company, even if he does not abet them at all, lest he be punished through them, as we have written many times in the name of Chazal. And according to what we see today, in our many sins, the issur of lashon hara having been blatantly violated, it befits everyone whose heart has been touched by fear of the L-rd not to sit in the company of men unless he knows them to be heedful of forbidden speech [as was the practice of the men of Jerusalem, as we find in Sanhedrin 23a]. For in most instances it is found that such company leads to lashon hara, rechiluth, and other [forms of] forbidden speech.

And, in truth, one who has an understanding heart, should flee the company of "street-corner men" as one flees fire. For why should he enter the company of men whom he will afterwards be obligated to reprove for their forbidden speech? For if he heard their words and neglects to rebut them, he, too, will be punished, as it is written in Sha’arei Teshuvah 197: If a man hears others speaking lashon hara or he hears "all mouths speaking foulness," or he sits in the midst of scoffers, who shame Torah and mitzvoth, and knowing them to be "defiers and despisers," who, if he reproves them, will not heed his words, wherefore, he puts his "hand to his mouth" — he, too, will be punished. For he will not have answered "the fools according to their folly," so that men may come to say of him that he, too, is like them and acquiesces in their words. How much more so is he obligated to answer and rebuke them to accord honor to the Torah and the mitzvoth which they have scorned and scoffed at, and to be jealous of the honor of the clean and the righteous whom they have spoken against. This is one of the reasons for which it is incumbent upon a man to leave the company of the wicked; for he will be punished for hearing their evil words and refraining from answering them. And this is explicitly stated by Solomon (viz. Proverbs 24:1-2)."

And see, my brother: If ten men are standing together and one of them is arrested for a certain offense that he was suspected of, and he is taken to court and tried, and he does not confess — and the other nine are also taken into custody for questioning in his case — though they consider themselves guiltless and have been remanded only to bring the first one's guilt to light, still their heart quakes within them and they are filled with remorse at having been "caught" in the company of the first, who is the cause of all this trouble. Then, afterwards, when with the help of the L-rd, they leave in peace without harm, they take great care not to be in his company again, lest the same thing happen to them. How much more so in our instance, when we know that in the world to come there will be punished both speaker and hearer and all who were with them in this evil company (viz. Chapter XIII in the name of Pirkei d'R. Eliezer) — how much more so must one flee this; and, when the yetzer hara entices him to join their company, to reflect: "It is enough for me that I must stand [in judgment] in the next world for my own sins. Why should I suffer punishment there for the sins of others!" In this way, the yetzer will be subdued.