Bhagavad Gita 2.15

यं हि न व्यथयन्त्येते पुरुषं पुरुषर्षभ।समदुःखसुखं धीरं सोऽमृतत्वाय कल्पते ॥२.१५॥

yaṃ hi na vyathayantyete puruṣaṃ puruṣarṣabhasamaduḥkhasukhaṃ dhīraṃ so’mṛtatvāya kalpate

That firm man, whom surely these afflictions do not, O chief among men, to whom pleasure and pain are the same, is fit for attaining immortality.

गीता