Bhagavad Gita · 5.21

बाह्यस्पर्शेष्वसक्तात्मा विन्दत्यात्मनि यत्सुखम्।स ब्रह्मयोगयुक्तात्मा सुखमक्षयमश्नुते॥

bāhya-sparśheṣhv asaktātmā vindaty ātmani yat sukham sa brahma-yoga-yuktātmā sukham akṣhayam aśhnute

One whose self is unattached to external sense pleasures discovers the happiness that is within the Self. Such a person, united with Brahman through yoga, enjoys imperishable happiness.

— Bhagavad Gita 5.21

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