Bhagavad Gita · 5.20

न प्रहृष्येत्प्रियं प्राप्य नोद्विजेत्प्राप्य चाप्रियम्।स्थिरबुद्धिरसम्मूढो ब्रह्मविद्ब्रह्मणि स्थितः॥

na prahṛṣhyet priyaṁ prāpya nodvijet prāpya chāpriyam sthira-buddhir asammūḍho brahma-vid brahmaṇi sthitaḥ

One should neither rejoice upon obtaining what is pleasant nor be disturbed upon receiving what is unpleasant. With steady intellect, free from delusion, the knower of Brahman is established in Brahman.

— Bhagavad Gita 5.20

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