You don't know the power of the dark side.
— Darth Vader (James Earl Jones)
4 seconds sound clip from the Star Wars: Episode VI - Return of the Jedi movie soundboard.
You can hear this line at 01:24:57.743 in the Blu-ray version of the movie.
Quote context
[...]
- Indeed you are powerful, as the Emperor has foreseen.
- Obi-Wan once thought as you do.
- You don't know the power of the dark side.
- I will not turn, and you'll be forced to kill me.
- Search your feelings, Father. You can't do this.
[...]