This quote is from his book, Happiness.
"Thoughts can be our best friends and our worst enemies. When they make us feel that the entire world is against us, every perception, every encounter, and the world's very existence become sources of torment. It is our thoughts themselves that rise up as enemies. They stampede through our mind in droves, each one creating its own little drama of ever increasing confusion. Nothing is right outside because nothing it's right inside."