Mbh 8.17.1c

Thus afflicted, all the people there regarded those two heroes as slain by Ashvatthama the warrior who had completely mastered the science of arms. Then the chief of the Dasharhas addressed Arjuna and said, "Why errest thou in thus sparing Ashvatthama? Slay this warrior. If treated with indifference, even this one will be the cause of great woe, like a disease not sought to be put down by treatment." Replying unto Keshava of unfading glory with the words "So be it!" Arjuna of unclouded understanding began with good care to mangle the son of Drona with his shafts. Now the son of Pandu, filled with rage, quickly pierced the massive arms, smeared with sandal-paste, and the chest, the head, and the unrivalled thighs of his antagonist with shafts equipped with heads like goats' ears, and shot with great force from Gandiva. Then cutting off the traces of Ashvatthama's steeds, Arjuna began to pierce the steeds themselves, whereat the latter bore Ashvatthama away to a great distance from the field. Thus borne away by these steeds endued with the speed of the wind, the intelligent son of Drona, deeply afflicted with the shafts of Partha, reflecting for some time, wished not to go back and renew the fight with Partha. Knowing that victory is ever with the chief of the Vrishnis and with Dhananjaya, that foremost one of Angirasa's race, endued with great activity, entered the army of Karna, deprived of hope and with shafts and weapons almost exhausted. Indeed, Drona's son, restraining his steeds, and having comforted himself a little, O sire, entered the force of Karna, teeming with cars and steeds and men. After Ashvatthama, that enemy of theirs, had been thus removed from the field by his steeds like a disease removed from the body by incantations and medicines and means, Keshava and Arjuna proceeded towards the samsaptakas, on their car whose rattle resembled the roar of the clouds and whose banner waved on the wind.'"