Mahabharata Sauptika Parva Chapter 1:3
"'Beholding that highly suggestive deed perpetrated in the night by the owl, Drona's son began to reflect on it, desirous of framing his own conduct by the light of that example. He said unto himself, "This owl teaches me a lesson in battle. Bent as I am upon the destruction of the foe, the time for the deed has come! The victorious Pandavas are incapable of being slain by me! They are possessed of might, endued with perseverance, sure of aim, and skilled in smiting. In the presence, however, of the king I have vowed to slay them. I have thus pledged myself to a self-destructive act, like an insect essaying to rush into a blazing fire! If I were to fight fairly with them, I shall, without doubt, have to lay down my life! By an act of guile, however, success may yet be mine and a great destruction may overtake my foes! People generally, as also those versed in the scriptures, always applaud those means which are certain over those which are uncertain. Whatever of censure and evil repute this act may provoke ought to be incurred by person that is observant of Kshatriya practices. The Pandavas of uncleansed souls have, at every step, perpetrated very ugly and censurable acts that are again fall of guile.
As regards this matter, certain ancient verses, full of truth, are heard, sung by truth-seeing and righteousness-observing persons, who sang them after a careful consideration of the demands of justice.
These verses are even these: 'The enemy's force, even when fatigued, or wounded with weapons, or employed in eating, or when retiring, or when resting within their camp, should be smitten. They should be dealt with in the same way when afflicted with sleep in the dead of night, or when reft of commanders, or when broken or when under the impression of an error.'"
"'Having reflected in this way, the valiant son of Drona formed the resolution of slaying during the night the slumbering Pandavas and the Pancalas. Having formed this wicked resolution and pledged himself repeatedly to its execution, he awoke both his maternal uncle and the chief of Bhojas. Awakened from sleep, those two illustrious and mighty persons, Kripa and the Bhoja chief, heard Ashvatthama's scheme. Filled with shame, both of them abstained from giving a suitable reply.
"'Having reflected for a short while, Ashvatthama said with tearful eyes, "King Duryodhana, that one hero of great might, for whose sake we were waging hostilities with the Pandavas, hath been slain! Deserted and alone, though he was the lord of eleven Akshauhinis of troops, that hero of unstained prowess hath been struck down by Bhimasena and a large number of wretches banded together in battle! Another wicked act hath been perpetrated by the vile Vrikodara, for the latter hath touched with his foot the head of a person whose coronal locks underwent the sacred bath! The Pancalas are uttering loud roars and cries and indulging in loud bursts of laughter.