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Hilchos Lashon Hara, Opening – Negative commands (3)
The speaker also transgresses the verse (Devarim 24:8), "Beware of the plague of tzara'as to guard exceedingly and to do." The Sifra explains that when the Torah says to "guard exceedingly," it means that one should not forget to be careful from lashon hara so that tzara'as does not afflict you because of these words.
Hilchos Lashon Hara, Opening – Negative commands (4)
The speaker and receiver also transgress the negative command (Vayikra 19:14), "Do not put a stumbling block before the blind." Because each one puts a stumbling block before his fellow by making him transgress explicit Torah prohibitions. But in this case there is a difference between the speaker and the receiver. The speaker transgresses this prohibition when the listeners are few or many, and on the contrary, the more listeners, the greater the prohibition. Not so the receiver who might transgress only if he is alone, since, if he did not listen the speaker would have no one to whom to relate the lashon hara. But if there are other listeners it may be that he may not transgress this prohibition but only the other negative commands mentioned in this opening. This is only if he came after the story began, But if he was there from the beginning he certainly transgresses in any case, because the prohibition began because of him.
Nonetheless, one must beware of such groups and not sit with them for in heaven they are written by the name of a wicked group. So we find in the will of Rabbi Eliezer Hagadol who adjured Hurkenus, his son: "My son, do not sit with groups that say bad of their fellows, because when the words rise above, they are written in a book, and all those standing there are called by the name of a wicked group."