Halakhah do Liczb 35:33

(16) And sometimes he transgresses also the issur of flattery, which is to many Geonim (e.g., HaRe'em, Baal Hatosfoth, and Hagaon R. Shlomoh ben G'virol) an absolute negative commandment, (viz. Bamidbar 35:33): "And you shall not flatter [the men of] the land." That is, if his intent in speaking lashon hara and rechiluth is to flatter the listener, whom he knows to bear hatred to the one spoken about, and thereby, to find favor in his eyes — an egregious sin — is it not enough that he does not fulfill the mitzvah of reproof (a positive commandment in the Torah), to reprove him for the hatred he bears his friend, that he also strengthens the hatred that already exists among them! And through him [the speaker], he [the listener] will persist in his wrong more and more, so that more quarrels and wrongs will result (G–d forbid)!
And know that this [the following] sin (in our many sins) is widespread. That is, when one speaks demeaningly of his friend, (often,) the listener, knowing that what is being said is unfounded, nevertheless nods his head [in agreement] and he, too, "smoothes over" the thing with his tongue, adding some words of taint. For the speaker is sometimes a man of means, or the like, from whom he receives favors, or who he fears will regard him as unwise, or the like [for remaining silent]. And, therefore, the yetzer will entice him, too, to concur in this. But know, my brother, that this, too, is essentially a transgression of the negative commandment of flattery — even if he adds but a few words — as is explained in the Be'er Mayim Chayim.
And in this regard it is written (Mishlei 23:2): "And put a knife to your throat [against speaking lashon hara] if you are a man of spirit." And one must rather expose himself to danger than bring his soul to such a sin. According to the Torah, every man under such circumstances must, in any event, strengthen himself not to abet him [the speaker] even by so much as one movement which would cause it to appear that he concurs with what he is saying. And in this connection we can understand the words of Chazal (Eiduyoth 5:6): "It is better to be called a fool all of one's days than to be wicked one moment before the Almighty." And this, even if he knows that his words of reproof will not be accepted by the speaker; for, otherwise, he certainly must reprove him for this, too (as will be explained, G–d willing, in Hilchoth Lashon Hara, Principle VI).