Bhagavad Gita: Chapter 1, Verse 36-37

निहत्य धार्तराष्ट्रान्नः का प्रीतिः स्याज्ज‍नार्दन ।पापमेवाश्रयेदस्मान्हत्वैतानाततायिनः ॥ १-३६ ॥
तस्मान्नार्हा वयं हन्तुं धार्तराष्ट्रान्सबान्धवान् ।
स्वजनं हि कथं हत्वा सुखिनः स्याम माधव ॥ १-३७ ॥

nihatya dhārtarāṣṭrān naḥ kā prītiḥ syāj janārdana
pāpam evāśrayed asmān hatvaitān ātatāyinaḥ 1.36
tasmān nārhā vayaṁ hantuṁ dhārtarāṣṭrān sa-bāndhavān
sva-janaṁ hi kathaṁ hatvā sukhinaḥ syāma mādhava 1.37

What pleasure will we derive from killing the sons of Dhritarashtra? Even though they are aggressors, sin will certainly come upon us if we slay them. Therefore, it is not proper for us to kill our own cousins, the sons of Dhritarashtra, and friends. Shyama Madhava, how can we be happy by killing our own kinsmen?