Bhagavad Gita: Chapter 2, Verse 56

दुःखेष्वनुद्विग्न‍मनाः सुखेषु विगतस्पृहः ।
वीतरागभयक्रोधः स्थिधीर्मुनिरुच्यते ॥ २-५६ ॥

duḥkheṣv anudvigna-manāḥ sukheṣu vigata-spṛhaḥ
vīta-rāga-bhaya-krodhaḥ sthita-dhīr munir ucyate 2.56

One who is not disturbed in mind even amidst misery or elated when there is happiness, and who is free from attachment, fear, and anger, is called a sage of steady mind.