Bhagavad Gita · 2.38

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

sukha-duḥkhe same kṛitvā lābhālābhau jayājayau tato yuddhāya yujyasva naivaṁ pāpam avāpsyasi

Treating pleasure and pain, gain and loss, victory and defeat alike, engage in battle. Thus you will not incur sin.

— Bhagavad Gita 2.38

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