Hilchos Lashon Hara chapter 10, section 16

All this shows how mistaken people are. Ask someone why he spoke lashon hara about another person, and he answers: "Because he spoke about me to so-and-so." This is a mistake.

First, if you heard that he spoke about you from the person he spoke to, you may not believe it, because it is forbidden to believe lashon hara.

And even if it is true, it is still forbidden to speak about him, as we said in paragraph 11.