Hilchos Lashon Hara, Opening – negative commands (16)
Sometimes one also transgresses the prohibition against flattery, which, according to many gaonim, is the absolute negative command of (Bamidbar 35:33), “You shall not do flattery in the land.” For example, one may have intent in one’s story to flatter the listener, knowing that he is a past enemy of the person he is speaking lashon hara about, and hoping through this to find favor in his eyes. Besides not fulfilling the mitzvah of rebuke by rebuking him for hating his fellow, he strengthens the hatred between them with his story.
Know also, that through our many sins it also happens that someone says lashon hara, and the listener, even though he knows it is lashon hara, nods his head as if agreeing to the lashon hara, and adds words so that he should get some future benefit from the speaker, or so that he should think he is clever. This itself, is the negative command of flattery since he is flattering the speaker.
Regarding this one can apply the verse (Mishlei 23:2), “And put a knife into your throat if you are a worthy person.” A person must even place himself in danger to not transgress such a prohibition. Regarding this one can apply the statement of the sages, “Better that one be called a madman all one’s life, and not be called wicked before the Omnipresent for one hour.”