Bhagavad Gita 2.15

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

yaṃ hi na vyathayantyete puruṣaṃ puruṣarṣabha |samaduḥkhasukhaṃ 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.

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

Our plain-language meaning of this verse is waiting for scholar review.