Hilchos Lashon Hara, chapter 1, section 1

It is forbidden to say something negative about another person - even if it is true. Chazal (our sages) call this "lashon hara."

The person who speaks it breaks a Torah prohibition (Vayikra 19:16): "Do not go as a talebearer (rachil) in your nation." Speaking badly about another person also counts as "rechilus" in this verse.