Bhagavad Gita: Chapter 2, Verse 33

अथ चेत्त्वमिमं धर्म्यं सङ्ग्रामं न करिष्यसि ।
ततः स्वधर्मं कीर्तिं च हित्वा पापमवाप्स्यसि ॥ २-३३ ॥

atha cet tvam imaṁ dharmyaṁ saṅgrāmaṁ na kariṣyasi
tataḥ sva-dharmaṁ kīrtiṁ ca hitvā pāpam avāpsyasi 2.33

If, however, you do not perform your religious duty of fighting, then you will certainly incur sins for neglecting your duties and thus lose your reputation as a fighter.