Hilchos Lashon Hara, chapter 1, section 4

Chazal say that for three sins one is punished in this world and does not have a portion in the next world. These are: idolatry, immorality, and bloodshed, while lashon hara is equal to them all [combined]. The Rishonim explain that Chazal refer to someone who is habituated to speak lashon hara constantly, due to it becoming as if permitted to him through much habit.