Bhagavad Gita 2.38

Look upon pleasure and pain, victory and defeat, with an equal eye. Be prepared for the combat, and you will commit no sin.

सुखदुःखे समे कृत्वा लाभालाभौ जयाजयौ।
ततो युद्धाय युज्यस्व नैवं पापमवाप्स्यसि॥

sukhaduḥkhe same kṛtvā lābhālābhau jayājayau|
tato yuddhāya yujyasva naivaṃ pāpamavāpsyasi||

English translation (above): Shri Purohit Swami (1935), public domain. Sanskrit text via github.com/gita/gita (Unlicense).