Bhagavad Gita Chapter 5, Verse 21

बाह्यस्पर्शेष्वसक्तात्मा विन्दत्यात्मनि यत्सुखम् | स ब्रह्मयोगयुक्तात्मा सुखमक्षयमश्नुते ||

bāhya-sparśeṣv asaktātmā vindaty ātmani yat sukham sa brahma-yoga-yuktātmā sukham akṣayam aśnute

One whose self is unattached to external contacts finds happiness in the Self. Such a person, whose self is engaged in union with Brahman, enjoys imperishable happiness.

Krishna describes the yogi's source of happiness: one whose self is unattached (asaktātmā) to external contacts (bāhya-sparśeṣu) finds (vindati) happiness (sukham) in the Self (ātmani). Such a person (saḥ), whose self is engaged in union with Brahman (brahma-yoga-yuktātmā), enjoys (aśnute) imperishable happiness (akṣayam sukham). This means the yogi doesn't depend on external objects for happiness—they find happiness within. External pleasures are temporary and depend on objects—the yogi finds happiness in the Self, which is imperishable. This is the practical outcome of karma yoga—happiness that doesn't depend on external circumstances.