Mbh 1.141.3a

“One day, Drona, addressing Arjuna before the assembled Kaurava princes, said, ‘There was a disciple of Agastya in the science of arms called Agnivesa. He was my preceptor and I, his disciple. By ascetic merit I obtained from him a weapon called Brahmasira which could never be futile and which was like unto thunder itself, capable of consuming the whole earth. That weapon, O Bharata, from what I have done, may now pass from disciple to disciple. While imparting it to me, my preceptor said, ‘O son of Bharadwaja, never shouldst thou hurl this weapon at any human being, especially at one who is of poor energy. Thou hast, O hero, obtained that celestial weapon. None else deserveth it. But obey the command of the Rishi (Agnivesa). And, look here, Arjuna, give me now the preceptorial fee in the presence of these thy cousins and relatives.’ When Arjuna, on hearing this, pledged his word that he would give what the preceptor demanded, the latter said, ‘O sinless one, thou must fight with me when I fight with thee.’ And that bull among the Kuru princes thereupon pledged his word unto Drona and touching his feet, went away northward. Then there arose a loud shout covering the whole earth bounded by her belt of seas to the effect that there was no bowman in the whole world like unto Arjuna. And, indeed, Dhananjaya, in encounters with the mace and the sword and on the chariot as also with the bow, acquired wonderful proficiency. Sahadeva obtained the whole science of morality and duties from (Vrihaspati) the spiritual chief of celestials, and continued to live under the control of his brothers. And Nakula, the favourite of his brothers taught by Drona, became known as a skilful warrior and a great car-warrior (Ati-ratha). Indeed, Arjuna and the other Pandava princes became so powerful that they slew in battle the great Sauvira who had performed a sacrifice extending over three years, undaunted by the raids of the Gandharvas.