Bhagavad Gita · 6.32

आत्मौपम्येन सर्वत्र समं पश्यति योऽर्जुन।सुखं वा यदि वा दुःखं स योगी परमो मतः॥

ātmaupamyena sarvatra samaṁ paśhyati yo 'rjuna sukhaṁ vā yadi vā duḥkhaṁ sa yogī paramo mataḥ

The yogi who, by comparison with oneself, sees equality everywhere — whether in happiness or suffering, O Arjuna — is considered the supreme yogi.

— Bhagavad Gita 6.32

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