Bhagavad Gita Chapter 1.36
pāpam evāśrayed asmān
hatvaitān ātatāyinaḥ
tasmān nārhā vayaḿ hantuḿ
dhārtarāṣṭrān sa-bāndhavān
sva-janaḿ hi kathaḿ hatvā
sukhinaḥ syāma mādhava
Translation
Sin will overcome us if we slay such aggressors. Therefore it is not proper for us to kill the sons of Dhritarashtra and our friends. What should we gain, O Krishna, husband of the goddess of fortune, and how could we be happy by killing our own kinsmen?