Halakha Imamat 19:14: Chafetz Chaim, Mishneh Torah, Robbery and Lost Property, Mishneh Torah, Torah Study & Shulchan Arukh, Choshen Mishpat

(2) He also transgresses (Devarim 27:18): "Cursed is he who misleads the blind man on the way," it being known that the intent of Scripture is to curse one who places a stumbling-block before another so that an issur be committed by him, as in the negative commandment (Vayikra 19:14): "And before the blind man do not place a stumbling-block," which we have already explained (Negative Commandments 4) as also falling into this category.

4) And both the speaker and the receiver transgress (Vayikra 19:14): "And before the blind man do not place a stumbling-block"; for each one [i.e., both the speaker and the listener] places a stumbling-block before his friend to transgress explicit negative commandments in the Torah. But there is a difference between the speaker and the listener in this regard. For the speaker transgresses this negative commandment both whether the listeners are many or few. Even more so, the more the listeners, the more he [the speaker] transgresses this negative commandment, placing a stumbling-block before many people. Not so the receiver. It is possible that he does not transgress this negative commandment unless he himself hears the lashon hara or the rechiluth from him [the speaker] at the moment, so that if he had left him, he would have no one to relate his lashon hara to. But if there are, besides him, different listeners at the time, it is possible that the hearer does not transgress this negative commandment, but only others mentioned in this introduction [see Be'er Mayim Chayim]. And all this, if he came after the "recital" had started. But the first listener — even though others arrived afterwards — certainly transgresses in all eight modes, for the issur was initiated through him. In any event, one must take great heed of such companions, not to sit with them; for "above" they are all inscribed as "a company of wickedness." And thus is it found in the will of R. Eliezer Hagadol to Hyrcanus, his son: "My son, do not sit with the companies of those who speak ill of their friends, for when their words rise on high, they are inscribed in a book, and all who stand there are described as "a wicked company."

It is forbidden to buy from a thief something stolen, and forbidden to dress it up with a modification so as to buy it. Anyone who does this, and other things like this, strengthens the hands of transgressors and [doing so] transgresses "do not put a stumbling block in front of the blind" (Lev. 19:14)