The Dark Knight (2008)
When The Batman got The Joker once and for all but didn’t kill him, The Joker was happy to have it confirmed that The Batman won’t kill him.
The Joker: [while hanging upside down] Oh, you. You just couldn’t let me go, could you? This is what happens when an unstoppable force meets an immovable object. You truly are incorruptible, aren’t you? You won’t kill me out of some misplaced sense of self-righteousness. And I won’t kill you because you’re just too much fun. I think you and I are destined to do this forever.