Bhagavad Gita: Chapter 2, Verse 15

यं हि न व्यथयन्त्येते पुरुषं पुरुषर्षभ ।
समदुःखसुखं धीरं सोऽमृतत्वाय कल्पते ॥ २-१५ ॥

yaṁ hi na vyathayanty ete puruṣaṁ puruṣarṣabha
sama-duḥkha-sukhaṁ dhīraṁ so ’mṛtatvāya kalpate 2.15

O best among men (Arjuna), the person who is not disturbed by happiness and distress and stable in all situations is certainly eligible for liberation.