Mbh 12.2.2a

"'Narada said, "It is even so, O mighty armed one, as thou sayest, O Bharata! Nothing could resist Karna and Arjuna in battle. This, O sinless one, that I am about to tell thee is unknown to the very gods. Listen to me, O mighty-armed one, as it befell in former days. How all the Kshatriyas, cleansed by weapons should attain to regions of bliss, was the question. For this, a child was conceived by Kunti in her maidenhood, capable of provoking a general war. Endued with great energy, that child came to have the status of a Suta. He subsequently acquired the science of weapons from the preceptor (Drona), that foremost descendant of Angirasa's race. Thinking of the might of Bhimasena, the quickness of Arjuna in the use of weapons, the intelligence of thyself, O king, the humility of the twins, the friendship, from earliest years, between Vasudeva and the wielder of Gandiva, and the affection of the people for you all, that young man burnt with envy. In early age he made friends with king Duryodhana, led by an accident and his own nature and the hate he bore towards you all. Beholding that Dhananjaya was superior to every one in the science of weapons, Karna one day approached Drona in private and said these words unto him, 'I desire to be acquainted with the Brahma weapon, with all its mantras and the power of withdrawing it, for I desire to fight Arjuna. Without doubt, the affection thou bearest to every one of thy pupils is equal to what thou bearest to thy own son. I pray that all the masters of the science of weapons may, through thy grace, regard me as one accomplished in weapons!' Thus addressed by him, Drona, from partiality for Phalguna, as also from his knowledge of the wickedness of Karna, said, 'None but a Brahmana, who has duly observed all vows, should be acquainted with the Brahma weapon, or a Kshatriya that has practised austere penances, and no other.