Bhagavad Gita · 1.35

अपि त्रैलोक्यराज्यस्य हेतोः किं नु महीकृते।निहत्य धार्तराष्ट्रान्नः का प्रीतिः स्याज्जनार्दन॥

api trailokya-rājyasya hetoḥ kiṁ nu mahī-kṛte ihatya dhārtarāṣhṭrān naḥ kā prītiḥ syāj janārdana

O Janardana, what pleasure could we derive from killing the sons of Dhritarashtra? Only sin would accrue to us by slaying these aggressors.

— Bhagavad Gita 1.35

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