Musar על ערכין 31:18

Even more so, should he take care not to speak to a shopkeeper about other shopkeepers or with any tradesman about his fellow tradesman. For, in all probability, one tradesman does not love the other and he is very likely to come through this to lashon hara. And if one knows that the first harbors hatred against the other, he should certainly not speak to him about the other. And, it goes without saying that he should not praise one to the other, which is certainly forbidden, as Chazal have said (Arachin 16a): "Let one not speak in praise of his friend, for from praising him he will come to demeaning him," all of which I have explained in Chafetz Chaim, Part One, Principle IX. And it is also highly advisable to avoid with him even speech in general, which is not in the class of demeaning or praising. For in his hatred of the other, he is highly likely to draw the speech in another direction, which will make it easier for him to shoot the arrows of his mouth against him.

In these parashiyoth, the Torah wrote at length about the greatness of the uncleanliness of the metzora [one afflicted with tzara'ath (leprosy)] and of his cleansing. And the Gemara in Arachin is well known, that tzara'ath afflicts one who speaks lashon hara, as stated there (Arachin 15b): "If one speaks lashon hara, he is afflicted with plague-spots, viz. (Psalms 101:5): 'He who slanders his neighbor in secret, him atzmith,' and (Vayikra 25:30): 'latzmituth,' which the Targum renders 'lachalutin'; [that is, that he be a metzora muchlat], concerning which we learned: 'The only difference between a quarantined leper and a confirmed [muchlat (similar to 'lachalutin')] leper is disheveling of the hair and rending of the clothes" [(these obtaining with the second, but not with the first)]. As to (Arachin 16a): "Plague-spots come for seven things, etc.", the Maharsha writes that there, it is possible that tzara'ath atones for him, for he is subject to quarantine — as opposed to the sin of lashon hara, where he is a metzora muchlat.

But there are times when the Sages permitted one to lie, for example, in order to make peace between one man and another (Yebamoth 65b). Similarly, one may praise a bride in the presence of the bridegroom and say that she is lovely and charming, even though she really is not (Kethuboth 17a). A guest (Arakin 16a) who has been well treated by the master of the house should not to say in front of many people, "How good that man is in whose house I was a guest, how much honor he paid me," lest many come to that host who are not worthy to be his guests, and concerning this it is said, "He that blesseth his friend with a loud voice, rising early in the morning, it shall be counted as a curse to him" (Prov. 27:14).

And there is another wrong which smacks of gossip — for example, where a person says, "Oh, better be silent about so and so; I don't want to say what I know about him!" — and so in all similar cases. And our Sages further said, "Let no man ever talk in praise of his neighbor, for through his praise he will come to disparage him" (Arakin 16a, and see Baba Bathra 164b). The meaning of this is: if you praise a man to his enemy, he will retort, "How can you praise him so much when he does this and this." And concerning this it is said, "He that blesseth his friend with a loud voice, rising early in the morning, it shall be counted a curse to him" (Prov. 27:14). But to praise a man before his friends is permitted, as we have learned, "Rabban Johanan the son of Zakkai had five (outstanding) disciples, and he used to recount their praises" (Aboth 2:8). Then, there is the one who speaks gossip by way of a joke or by way of frivolity (that is, he is not speaking out of hatred), and that is what Solomon said in his wisdom, "As a madman who casteth firebrands, arrows and death; so is the man that deceiveth his neighbour and saith: "Am not I in sport?" (Prov. 26:18—19).

Then there is he who speaks gossip by way of deceit: he tells it with seeming innocence, as though he does not know that he is indulging in gossip, and when others rebuke him he says, "I really don't know whether so and so is guilty of these things." Or he says, "This may be merely gossip." One who speaks words that cause harm to his fellow man, whether it be to his body or to his money, even though it be to distress him or to frighten him, it is gossip. If a man says something to his companion, he is forbidden to reveal it without his permission (Yoma 4b). But anything which a man says in front of three people it is as though he intended it to be common knowledge and if one of the three who heard it told about it we cannot say that this is gossip (Arakin 15b). But if the teller intends to reveal more than he heard, then there is something of gossip in it. And if the speaker warns those who heard him not to reveal it, even though he speaks in the presence of many people, still if one of those who were warned does reveal it, it is a sort of gossip. There is a story about a certain pupil who revealed a thing that had been said in the house of study twenty-two years earlier and they drove him out of the house of study and they said, "This one is a revealer of secrets" (Sanh. 31a).