Srimad Bhagvata Mahapurana: Book 1: Chapter 7: Verses 33-48
Then he quickly seized the hard-hearted son of Krpi, his eyes burning with rage, and bound him with a rope as one would bind a sacrificial animal. Having forcibly bound the enemy with a rope, he was about to take him to his camp, when the lotus-eyed Lord said to him in rage:- "Arjuna (son of Prtha), you ought not to spare him. Do kill this fallen Brahmana, who murdered at dead of night innocent boys buried in sleep. A pious man would never kill an enemy who is drunk, unguarded, insane, asleep, tender of age, stupid, terror-stricken or fallen at his feet, nor would he kill a woman nor one who has lost one's chariot. But to slay the merciless wretch who maintains his own life at the cost of another's is to do him good; for the man would (otherwise) repeat such crimes and thereby precipitate his fall. Moreover, you gave your word to Draupadi (the daughter of the king of Pancala) within my hearing: 'I shall bring you the head of the man who has slain your sons, 0 proud lady. Therefore, O valiant Arjuna, dispose of this sinful ruffian, who has murdered your own sons. This unworthy member of his race has done something which was loathsome even to his master (Duryodhana)." Urged in these words by Sri Krsna who was putting his righteousness to the test, the great Arjuna (the son of Kunti) would not slay Aswatthama (his preceptor's son), even though he had murdered his own sons.
Then, coming up to his camp, Arjuna, who had Sri Govinda for his friend and charioteer, showed him (Aswatthama) to his beloved consort, who had been grieving all the while for her murdered sons. Looking intently at Aswatthama (Drona's son), who had been brought in that plight, bound with cords like a sacrificial beast, his face cast down on account of his shameful act, Draupadi,who was tender by nature, took compassion on the offender and bowed to him. That noble lady could not bear to see him brought in bondage and said, "Let him be freed, let him be released,a Brahmana that he is and worthy of our utmost adoration. That worshipful Drona, by whose grace you learnt the science of archery with its manifold secrets and acquired the knowledge of the various missiles as well as how to discharge and call them back-it is he himself who is present in the person of his progeny, his wife, his better half, Krpi (a sister of Acarya Krpa, another preceptor of the Pandavas) yet survives : she did not follow him (declined to ascend his funeral pile) only because of her having given birth to a valiant son. Therefore, my blessed lord, the family of your preceptor, which is ever worthy of adoration and salutation for you, does not deserve persecution at your hands, pious as you are. Let not his (Aswatthama's) mother, Gautami ( a descendant of the sage Gautama), who worshipped her husband as a deity, wail even as I do, grieved at the loss of my children and shedding tears again and again. The whole family of those princes of unconquered mind by whom the Brahmana race is angered is plunged in grief and is soon burnt up with all their kith and kin."