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. A
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.