The Yoga Sutras of Pantanjali.

There is a sanskrit saying, “Mana eva manushyanam karanam bandha mokshayoho.” “As the mind, so the man; bondage or liberation are in your own mind.” If you feel bound, you are bound. If you feel liberated, you are liberated. Things outside neither bind nor liberate you; only your attitude toward them does…. There’s nothing wrong with the world. You can make it a heaven or a hell according to your approach. That is why the entire Yoga is based on chitta vritti nirodhah (restraint of the mind-stuff). If you control your mind, you have controlled everything. Then there is nothing in the world to bind you.

— The Yoga Sutras of Pantanjali. From the second sutra, translated by Sri Swami Satchidananda