Hilchos Lashon Hara, chapter 1, section 4

Chazal say that for three sins a person is punished in this world and has no share in the World to Come: idol worship, immorality and murder. And lashon hara is equal to all three of them together.

The Rishonim (early authorities) explain that Chazal mean someone who speaks lashon hara all the time - because from so much habit, it has become to him as if it were allowed.