Hilchos Rechilus, chapter 3, section 2

It is rechilus to say to anyone that Reuven spoke against Shimon, because the story will
spread and will cause enmity between Reuven and Shimon. Even if one warns the person one
speaks to not to spread the story, and even if one knows for sure that he will not spread the story,
it is still forbidden, because merely saying that Reuven spoke against Shimon is itself generally
lashon hora.