Hilchos Rechilus, chapter 3, section 1

It is forbidden to speak rechilus even if it is absolutely true, even if the person spoken of is not present, and even if one knows that one would have been willing to speak it right in front of him. It is even worse to brazenly say in front of him, “You spoke such and such about so and so,” “You did such and such to so and so,” for this generates even more hatred as the person he spoke about is certain it must be true. For if not, the person speaking rechilus would not have had the nerve to say in front of the person that he did the wrongdoing.