Bhagavad Gita · 2.15

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

yaṁ hi na vyathayanty ete puruṣhaṁ puruṣharṣhabha sama-duḥkha-sukhaṁ dhīraṁ so 'mṛtatvāya kalpate

O best among men, the person whom these do not disturb, who remains equal in pain and pleasure, and is steady — such a person is fit for immortality.

— Bhagavad Gita 2.15

Source: Sacred-Texts.com (Sanskrit) + Bhagavad-Gita.io (translations)