Hilchos Lashon Hara, chapter 1, section 1

It is forbidden to speak negatively of one’s fellow, even if the information is true. This is what Chazal call lashon hara, and the person who speaks it transgresses the negative command (Leviticus 19:16), “Do not go as a talebearer (rachil) in your nation.” To speak negatively of one’s fellow is regarded as rechilus.