Hilchos Rechilus, chapter 1, section 1 2
This chapter will explain the prohibition against speaking rechilus even if it is completely true, what rechilus comprises, what to answer if someone asked what another person said about him, and all other details of the prohibition.
1) Speaking rechilus contravenes the negative command, Do not go tale bearing in your nation. Because this is a great sin and causes the death of many Jews, the Torah then writes, Do not stand by the blood of your fellow. See how the rechilus of Do’eg the Edomite caused the death of the people of Nov, the town of kohens. Besides this explicit negative command, one also transgresses many other negative and positive commands as explained in the introduction.
2) What is rechilus? To carry words from one person to another saying, “So and so said such and such about you,” “He did such and such to you,” “I heard that he did such and such to you, or wants to do it.” Even if the thing he speaks of is not derogatory even in the speaker’s opinion, and even if the person blamed would admit saying or doing this thing if asked, either because he was correct in acting like this or because he had a justifiable intention in what he did, even so, this is rechilus.