Bhagavad Gita Chapter 5, Verse 20

न प्रहृष्येत्प्रियं प्राप्य नोद्विजेत्प्राप्य चाप्रियम् | स्थिरबुद्धिरसम्मूढो ब्रह्मविद्ब्रह्मणि स्थितः ||

na prahṛṣyet priyaṁ prāpya nodvijet prāpya cāpriyam sthira-buddhir asammūḍho brahma-vid brahmaṇi sthitaḥ

One with steady intellect, not deluded, who knows Brahman and is established in Brahman, does not rejoice on obtaining what is pleasant, nor is agitated on obtaining what is unpleasant.

Krishna describes the yogi who is established in Brahman: one with steady intellect (sthira-buddhiḥ), not deluded (asammūḍhaḥ), who knows Brahman (brahma-vit) and is established (sthitaḥ) in Brahman (brahmaṇi). Such a person does not rejoice (na prahṛṣyet) on obtaining what is pleasant (priyaṁ prāpya), nor is agitated (na udvijet) on obtaining what is unpleasant (apriyam prāpya). This is the practical sign of being established in Brahman—equanimity in pleasure and pain. The yogi doesn't get excited by success or depressed by failure—they remain steady because they're established in Brahman, which is beyond dualities. This is the state of true karma yoga—acting in the world while remaining unaffected by its dualities.