Bhagavad Gita 1.37 Chapter 1, Verse 37

तस्मान्नार्हा वयं हन्तुं धार्तराष्ट्रान्स्वबान्धवान् ।
स्वजनं हि कथं हत्वा सुखिनः स्याम माधव ॥१.३७॥

tasmānnārhā vayaṃ hantuṃ dhārtarāṣṭrānsvabāndhavān
svajanaṃ hi kathaṃ hatvā sukhinaḥ syāma mādhava

Meaning

So it is not right for us to kill our own kinsmen, the sons of Dhritarashtra. O Madhava, how could we be happy after killing our own people?

Newly written by Bhajan Dhun — scholarly review in progress.

Reference: Bhagavad Gita 1.37 (Chapter 1, Arjuna's Despair (Arjuna Vishada Yoga), Verse 37)