Hilchos Lashon Hara chapter 6, section 1
The Torah forbids believing lashon hara that someone sinned against man or God. That is, one may not believe the story is true for this disparages the victim in one's eyes. This applies even if one does not verbally agree to the story - for if one did one would sin doubly by both speaking and accepting the story. Someone who believes lashon hara transgresses the negative command, Do not accept a false report, which warns against this sin according to the Mechilta. In addition, one transgresses the other positive and negative commands mentioned in the introduction.
Furthermore, the sages say that whoever believes lashon hara deserves to be thrown to the dogs because juxtaposed to verse Do not accept a false report is the verse You shall throw it to the dogs. They also say that someone who believes lashon hara is punished more than the one who speaks it.
Hilchos Lashon Hara chapter 6, section 2
The Torah forbids listening to lashon hara even if one does not intend to believe it. However there are a number of differences between listening and believing. Listening is only forbidden if there is no benefit. If one understands from the beginning of the story that the speaker wants to show that a person is dishonest or suchlike and one was thinking of doing business or arranging a marriage with his family and suchlike, one may listen to the story with intent to suspect it may be true and be careful of him. For one's intent is not to hear of his denigration but to save oneself from controversy or damage. The same applies if listening to the story will benefit others. For example, one may intend to rebuke the person spoken of for the sin he committed, return what he stole, or appease whoever he insulted. However, to actually believe the story is always forbidden.
