Bhagavad Gita: Chapter 2, Verse 38

सुखदुःखे समे कृत्वा लाभालाभौ जयाजयौ ।
ततो युद्धाय युज्यस्व नैवं पापमवाप्स्यसि ॥ २-३८ ॥

sukha-duḥkhe same kṛtvā lābhālābhau jayājayau
tato yuddhāya yujyasva naivaṁ pāpam avāpsyasi 2.38

Without considering happiness or distress, loss or gain, victory or defeat, fight for the sake of fighting. This way you will never incur sin.