Bhagavad Gita 2.65

Having attained peace, he becomes free from misery; for when the mind gains peace, right discrimination follows.

प्रसादे सर्वदुःखानां हानिरस्योपजायते।
प्रसन्नचेतसो ह्याशु बुद्धिः पर्यवतिष्ठते॥

prasāde sarvaduḥkhānāṃ hānirasyopajāyate|
prasannacetaso hyāśu buddhiḥ paryavatiṣṭhate||

English translation (above): Shri Purohit Swami (1935), public domain. Sanskrit text via github.com/gita/gita (Unlicense).