Mbh 10.12.1

Vaishampayana said, "After the irresistible Bhimasena had set out, that bull of Yadu's race, possessed of eyes like lotus-petals, addressed Kuru's son Yudhishthira, saying, 'O son of Pandu, this brother of thine, overwhelmed with grief at the slaughter of his sons, proceedeth alone to battle, from desire of slaying the son of Drona. O bull of Bharata's race, of all thy brothers, Bhima is thy dearest! Beholding him fallen into a great danger why dost thou not stir thyself? The weapon called brahmashira, which that subjugator of hostile towns, Drona, communicated to his son, is capable of consuming the whole world. The illustrious and highly blessed preceptor, that foremost of all wielders of bows, delighted with Dhananjaya, had given him that very weapon. Unable to endure it, his only son then begged it of him. Unwillingly he imparted the knowledge of that weapon to Ashvatthama. The illustrious Drona knew the restlessness of his son. Acquainted with all duties, the preceptor laid this command on him, saying, "Even when overtaken by the greatest danger, O child in the midst of battle, thou shouldst never use this weapon, particularly against human beings." Even thus the preceptor Drona spoke unto his son. A little while after he again spoke, saying, "O bull among men, thou wilt not, it seems, walk in the path of the righteous." Hearing those bitter words of his sire, the wicked-souled Ashvatthama, in despair of obtaining every kind of prosperity, began in grief to wander over the earth.