Bhagavad Gita · 2.65

प्रसादे सर्वदुःखानां हानिरस्योपजायते।प्रसन्नचेतसो ह्याशु बुद्धिः पर्यवतिष्ठते॥

prasāde sarva-duḥkhānāṁ hānir asyopajāyate prasanna-chetaso hy āśhu buddhiḥ paryavatiṣhṭhate

In that tranquility, all sorrows are destroyed. The intellect of one whose mind is tranquil soon becomes firmly established.

— Bhagavad Gita 2.65

Source: Sacred-Texts.com (Sanskrit) + Bhagavad-Gita.io (translations)