Bhagavad Gita 2.15

The hero whose soul is unmoved by circumstances, who accepts pleasure and pain with equanimity, is only fit for immortality.

यं हि न व्यथयन्त्येते पुरुषं पुरुषर्षभ।
समदुःखसुखं धीरं सोऽमृतत्वाय कल्पते॥

yaṃ hi na vyathayantyete puruṣaṃ puruṣarṣabha|
samaduḥkhasukhaṃ dhīraṃ so'mṛtatvāya kalpate||

English translation (above): Shri Purohit Swami (1935), public domain. Sanskrit text via github.com/gita/gita (Unlicense).