Gemara

(Rav Sheshes): One who speaks or believes Lashon ha'Ra or testifies falsely should be cast to dogs. Right after "la'Kelev Tashlichun Oso", it says "Lo Sisa Shema Shav."

Nidah 61a (Beraisa) Question: Why does it say "Anashim Asher Hikah b'Yad Gedalyah"? Gedalyah did not kill them. Yishmael killed them!

Answer: Gedalyah should have been concerned for Yochanan's words (that Yishmael seeks to kill Gedalyah). He was not (and this enabled Yishmael to kill Gedalyah and them), so their death is attributed to Gedalyah.

(Rava): Even though one may not accept (believe) Lashon ha'Ra, he should be concerned lest it is true.

There were rumors that some Galileans had killed someone. They asked R. Tarfon to hide them.

R. Tarfon: If I will not hide you, perhaps you will be found (and executed). However, I cannot hide you. Chachamim taught that one must be concerned for Lashon ha'Ra! Go hide yourselves!

Erchin 15b (Chachamim of Eretz Yisrael): Lashon Tilsai kills three people - the speaker, the listener and the subject (the one spoken about).

Kesuvos 5a (Bar Kapara): "You should have a shovel with Azenecha (your weapons)" - we can read this like Oznecha (your ears), to teach that one should stick his finger in his ear if he hears something improper.

5b (Beraisa): One should not let his ears hear idle words, for they are the first limbs to be singed (in Gehinom).

Mechilta (Mishpatim 20): "Lo Sisa Shema Shav" is a Lav forbidding accepting Lashon ha'Ra. Alternatively, it forbids a judge to hear a party's claim in the absence of the other party.

Rishonim

Rambam (Hilchos De'os 7:2): A Rachil is one who goes around saying 'Ploni said such and such, I heard such and such about Ploni.' He destroys the world, even if it is true. Worse than this is Lashon ha'Ra, which is seeking detriment about another, even if it is true.

Rebuttal (Ra'avad): The former is worse, for it is triple and kills. The latter is double and kills only himself.

Kesef Mishneh: The Ra'avad learns from Erchin that Lashon ha'Ra kills three when it is Rechilus. When Shimon hears what Ploni intends to do to him, he kills Ploni. Ploni's Go'alei ha'Dam (relatives) kill Shimon to avenge the murder, and those who seek to avenge the blood of both of them kill the teller, who caused both deaths. One who hears mere detriment is not punished, for he is not so impressed by what was said, and there is no concern lest the subject kill the listener. He harms only himself and the subject. The Ra'avad says that he kills only himself, i.e. but not the listener. The Rambam holds that the Gemara discusses Lashon ha'Ra, for it kills all three (like I described above regarding Rechilus).

Be'er Mayim Chayim (in Chafetz Chayim 6:1): The Kesef Mishneh only discusses which Lashon ha'Ra kills. Surely it is forbidden to hear also regular Lashon ha'Ra. One cannot say that whenever the Gemara forbids accepting Lashon ha'Ra, it refers to Rechilus!

Radvaz (Leshonos ha'Rambam 1, brought in Likutim in Frankel Rambam): The Rambam says that the latter is worse because he sins without any benefit. The former expects to be rewarded for his tale, like Do'eg and the Zifim expected Sha'ul to reward them. Even though it is severe in another sense, it is not as severe as Lashon ha'Ra, in which the teller does not expect to get any benefit - "v"Ein Yisron l'Va'al ha'Lashon."

Rambam (3): Lashon ha'Ra kills three people - the speaker, the one who accepts it, and the subject. The one who accepts it is (punished) more than the one who told it.

Rambam (6): One may not live in the vicinity of Ba'alei Lashon ha'Ra, and all the more so one may not sit with them and hear their words.

Sha'arei Teshuvah (3:213): It says "Leshon Sheker Yisna Dakav." A humble person of broken spirit hates Sheker, and he does not agree to hear it. He desires the honor of people, and not their shame. "Lo Sisa Shema Shav" forbids believing in the heart that the tale is true and to despise the subject.

Sha'arei Teshuvah (225): We are commanded from the Torah not to accept Lashon ha'Ra.

Be'er Mayim Chayim (1,2): R. Yonah connotes that if he sees that if he believes that Ploni did this, he will have no side to justify Ploni, he is commanded not to believe it, lest Ploni be disgraced in his eyes. However, if there is a side of Zechus to say that Ploni did properly, or he did not know the Isur, or if Ploni said something, he did not mean like the teller says, we need not think that the take is Sheker, just we may not look down on Ploni due to it. It is not clear whether the Isur to listen to Lashon ha'Ra without accepting it is mid'Oraisa. The Rambam forbids hearing Lashon ha'Ra, but perhaps this is only mid'Rabanan. R. Yonah forbids only believing, but perhaps he teaches that even when one may listen, e.g. to guard himself, he may not believe it. In many places Chazal forbade accepting Lashon ha'Ra, but perhaps they taught what is clearcut (always forbidden), for sometimes one may hear it.

Rosh (Nidah 9:5): Rashi explains that R. Tarfon said that he cannot hide the people, for one must be concerned for Lashon ha'Ra; perhaps they killed, and it is forbidden to hide them. Surely, this concern does not forbid saving them! Rather, the She'altos (Vayeshev) explains that if they really killed, one who helps them would be liable to the king, who forbade aiding murderers. One may be concerned for Lashon ha'Ra only if it can cause harm to himself or others.

Poskim

Chafetz Chayim (6:1): The Torah forbids accepting Lashon ha'Ra, whether it is about a matter Bein Adam l'Makom or Bein Adam l'Chaveiro. One may not believe in his heart that it is true, for through this the subject is disgraced in his eyes. This is even if he does not explicitly agree to the tale. If he agrees, he doubles the Aveirah, for he tells and accepts. One who accepts transgresses "Lo Sisa Shema Shav", and other Lavim, like I wrote in the introduction to this Sefer.

Chafetz Chayim (2): There is an Isur Torah even for hearing Lashon ha'Ra, even if he does not intend to accept the matter, since he leans his ear to listen. However, there are many differences between hearing and accepting. One may hear if it will be relevant for him. E.g. if the teller wants to show him that Ploni is not trustworthy, and he had intended to engage in business with him, become a partner with him or make a Shiduch with him, l'Chatchilah he should listen to be concerned and guard himself. He does not intend to hear detriment, rather, just to spare himself harm and quarrels. The same applies if he will not benefit from hearing, but he will be able to help others, e.g. to rebuke Ploni for this, and perhaps this will cause him to repent or return what he stole. However, in every case it is forbidden to accept, i.e. to decide in his heart that the matter is true.

Chafetz Chayim (3): To fulfill his Chiyuv b'Yedei Shamayim, if David comes to him and wants to talk about someone, and he understands that David intends to say detriment, he should ask from the beginning if the tale is relevant to him for the future, or if he will be able to fix it through rebuke, etc. If it is, he may listen but not believe it until it will be clarified. If he understands that there is no benefit, and it is just slander, he may not even listen.

Chafetz Chayim (4): Sometimes it is a Mitzvah to hear someone say detriment about another, e.g. if he estimates that through hearing the entire matter, he will be able to show the teller or other listeners that the event was not like it was told, or other matters of Zechus. If one stumbled and heard Lashon ha'Ra (without justification), he should immediately seek Zechus for the subject and try to convince the teller. Through this, he retroactively fixes his Isur of listening. However, if he knows the teller's nature, and whatever he says the teller will merely say more detriment about the subject, it is better to be silent. After the teller leaves, it is a Mitzvah to defend the subject in the eyes of the other listeners

Chafetz Chayim (5): If one was sitting among people, and they started speaking Lashon ha'Ra, and he estimates that rebuke would not be heeded, if he can leave or stick his finger in his ear, this is a great Mitzvah. If he cannot leave, and it is hard to stick his finger in his ear lest they mock him, he must envigorate himself not to transgress hearing and accepting Lashon ha'Ra. He must resolve absolutely not to believe it; he may not enjoy the tales; and he may not show in any way that he agrees with their words. Rather, he sits like a rock. It is better if he can frown, to show that he does not agree to their folly.