For years I lived with the impression that I have heard that quote somewhere in the movies but I can't Google it so I am starting to think I am confused. Is there an instance where Yoda (or maybe another character) says that?
The phrase you thought in scripts occur does not.
I looked for the phrase "dark side is" in all the scripts that can be downloaded from https://starwarssuperfans.wordpress.com/real-world-news/star-wars-scripts/ .
The words do occur a couple of times, but not in combination with "powerful". The only references are the ones already mentioned in comments.