Hilchos Rechilus, chapter 3, section 2

It is rechilus to tell anyone that Reuven spoke against Shimon. The story will spread and cause hatred between Reuven and Shimon.

This is forbidden even if you warn the listener not to pass it on, and even if you know for sure he will not pass it on. Simply saying that Reuven spoke against Shimon is usually lashon hara in itself.