Rambam(Hilchos De'os 7:1) states that the Torah prohibits rechilus (gossip). But he goes on to say (Hilchos De'os 7:2): "There is a much more serious sin than rechilus (gossip) which is included in this prohibition and that is lashon harah. Lashon harah is saying negative things about another person – even though they are true.... The Ravad disagrees with the Rambam and states that lashon harah is not a more serious sin than rechilus – the opposite is true.
Rav Asher Weiss (Minchas Asher Vayikra #41) notes that the Rambam seems to be learning the prohibition of lashon harah is learned from rechilus by a kal v'chomer. In fact he says that the Chofetz Chaim(Be'er Mayim Chaim 1:4) states that according to the Ravad's view that lashon harah is not as serious a sin as rechilus that means that it can't be learned by a kal v'chomer. So how does the Ravad learn the prohibition of lashon harah? The Chofetz Chaim suggests that the Ravad learns lashon harah from verses other than those dealing with rechilus. However Rav Weiss disagrees and notes that there is no problem because both the Rambam and the Ravad agree that lashon harah is learned from the verse of rechilus. The Rambam doesn't say that there is a kal v'chomer but rather says that lashon harah is "included in the prohibition " of rechilus. Therefore the dispute is only which of the prohibited speach learned from Vayikra(19:16) is a more serious sin.