NUM426 At times, one also violates the prohibition of flattery, which according to many Ge’onim (namely, the Tosafist Rabbi Eliezer of Metz, the Gaon, and Rabbi Shlomo ben Gevirol) is considered a full-fledged Torah prohibition, derived from the pasuk: וְלֹא־תַחֲנִ֣יפוּ אֶת־הָאָ֗רֶץ  “Do not corrupt the land” (Bamidbar 35:33). For example, if one speaks lashon hara or rechilus with the intention of flattering the listener--for he knows that this listener already hates the person being spoken about, and he hopes to find favor in the listener's eyes by disparaging that person—then he transgresses this prohibition. This is a terrible sin, for not only does the speaker fail to rebuke the listener for hating the other person--in fulfillment of the mitzvah of giving rebuke (which is a positive Torah commandment--his words also reinforce the hatred that already exists between the two. Because of him, the listener will repeat his sinful actions over and over again, and a new dispute will arise, as will several other problems; Hashem should save us. Note that unfortunately, this prohibition is violated very frequently. For instance, when someone speaks disparagingly of another person, then even though the listener realizes that what we said was against halachah, he might nevertheless nod his head in agreement. He might also embellish the story by adding a few negative words of his own, because at times the speaker is a prominent person from whom the listener has received favors, or because the listener is afraid that if he does not speak up he will be considered unintelligent or the like. The yetzer hara will therefore be able to convince him to yield to the pressure and show his approval to the speaker. However, my brother, you should know that this is essentially a violation of the prohibition of flattery, even if the listener adds only a few words. It is regarding such circumstances that the pasuk says: וְשַׂמְתָּ֣ שַׂכִּ֣ין בְּלֹעֶ֑ךָ אִם־בַּ֖עַל נֶ֣פֶשׁ אָֽתָּה׃ “And you shall place a knife in your throat if you are a man of spirit” (Mishlei 23:2). A person is obligated to endanger his life rather than incur such a grievous sin. According to the Torah, if a person finds himself in such a situation he must, at minimum, exert a great deal of self-control and ensure that he does not encourage the speaker by making even one gesture that indicates that he approves of his words. It is about situations like these that Chazal said (Eduyos 5:2): “Better that a person be called a fool his entire life than be considered wicked in the eyes of Hashem for even one moment.” This is relevant even when one knows that the speaker will not be influenced by his rebuke. However, if there is a possibility that his rebuke may be effective, then he is certainly obligated to reprove the speaker as well...