Sūta Gosvāmī said: Although Kṛṣṇa, who was examining Arjuna in religion, encouraged Arjuna to kill the son of Droṇācārya, Arjuna, a great soul, did not like the idea of killing him, although Aśvatthāmā was a heinous murderer of Arjuna’s family members.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
एवं परीक्षता धर्मं पार्थः कृष्णेन चोदितः । नैच्छद्धन्तुं गुरुसुतं यद्यप्यात्महनं महान्
evaṃ parīkṣatā dharmaṃ pārthaḥ kṛṣṇena coditaḥ | naicchaddhantuṃ gurusutaṃ yadyapyātmahanaṃ mahān
SB 1.7.40