Bhagavad Gita 6.32
O Arjuna! He is the perfect saint who, having been taught by the likeness within himself, sees the same Self everywhere, regardless of whether the outer form be pleasurable or painful.
आत्मौपम्येन सर्वत्र समं पश्यति योऽर्जुन।
सुखं वा यदि वा दुःखं सः योगी परमो मतः॥ātmaupamyena sarvatra samaṃ paśyati yo'rjuna|
sukhaṃ vā yadi vā duḥkhaṃ saḥ yogī paramo mataḥ||
English translation (above): Shri Purohit Swami (1935), public domain. Sanskrit text via github.com/gita/gita (Unlicense).