It seems like you're referencing a specific scene from "Star Wars: Episode III - Revenge of the Sith." However, the actual dialogue goes differently. When Obi-Wan warns Anakin, he's actually advising him during a climactic duel, but the focus is usually on Anakin's overconfidence rather than a rock.
If you want a fun twist on this, you could imagine Obi-Wan saying something humorous about the dangers of the environment during their fight:
"Anakin, don't underestimate the force of gravity! Remember what happened the last time you slipped on a rock!"
This adds a light-hearted spin to the intense moment. Would you like to explore more "Star Wars" concepts or scenes?