Chapter 2 · The Yoga of Knowledge · Krishna said
Bhagavad Gita 2.15 — Steady in pleasure and pain
यं हि न व्यथयन्त्येते पुरुषं पुरुषर्षभ। समदुःखसुखं धीरं सोऽमृतत्वाय कल्पते ॥
yaṃ hi na vyathayantyete puruṣaṃ puruṣarṣabha | samaduḥkhasukhaṃ dhīraṃ so'mṛtatvāya kalpate ||
Meaning
The steady person who is not shaken by these, who stays the same in pain and pleasure, becomes fit for immortality.