Bhagavad Gita · 1.36

पापमेवाश्रयेदस्मान्हत्वैतानाततायिनः।तस्मान्नार्हा वयं हन्तुं धार्तराष्ट्रान्स्वबान्धवान्।स्वजनं हि कथं हत्वा सुखिनः स्याम माधव॥

pāpam evāśhrayed asmān hatvaitān ātatāyinaḥ tasmān nārhā vayaṁ hantuṁ dhārtarāṣhṭrān sa-bāndhavān sva-janaṁ hi kathaṁ hatvā sukhinaḥ syāma mādhava

Sin alone would be ours if we were to slay these aggressors. Therefore, it is not right for us to kill the sons of Dhritarashtra and our relatives. O Madhava, how could we be happy by killing our own kinsmen?

— Bhagavad Gita 1.36

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