Musar su Proverbi 23:9

Furthermore, he should take great care not to stand in the company of men. And even if he needs something essential, he should not linger there, unless he does not know them to be men among whom there is no forbidden speech. And even if the entire company are good men, and there is found one among them who is a man of malice, he spoils the entire company and it is advisable to leave them, or at least to court silence at that time with all of his strength, as written in Rosh Hagivah: "Take heed, my son, of what was said by King Solomon, may peace be upon him (Mishlei 23:9): 'In the ears of a fool do not speak, for he will shame the wisdom of your words.' Take heed, then, of a gathering of a hundred men if there is among them one fool or one speaker of levity, especially one who is wise in his [own] eyes, who is the true fool, as it is written: 'The fool is wise in his eyes.' Strengthen yourself and sit in silence and speak of nothing in that gathering. For even if you speak all kinds of wisdom, he will triumph over you, and you will take [only] shame for yourself. For thus has Solomon said (Ibid. 11:2): '[The man of] malice comes and shames comes, etc.' And even if you want to speak [privately] with your friend, make sure that you are not heard by that man, even from behind a fence. For this reason it is written 'In the ears of a fool do not speak,' and not 'With a fool do not speak.' This is the voice of experience. I have tried all of this several times at a mitzvah feast and have found it to be true. Therefore, flee, my son, from a gathering like this, for he [the fool] demeans all who are there; or seize silence and be saved."