He became so powerful, the only thing he was afraid of was losing his power. Which eventually, of course, he did.
13 seconds sound clip from the Star Wars: Episode III - Revenge of the Sith movie soundboard.
You can hear this line at 00:47:16 in the Blu-ray version of the movie.
Quote context
[...]
- He had such a knowledge of the dark side, he could even keep the ones he cared about from dying.
- He could actually save people from death?
- The dark side of the Force is the path way to many abilities some consider to be unnatural.
- It's ironic. He could save others from death, but not himself.
- Is it possible to learn this power?
- All Red units, over here! Let's go, let's go, let's go!
- Palpatine thinks General Grievous is on Utapau?
[...]