Mbh 7.81.1b

And as they approached the snakes, conversant as they were with the Vedas, they uttered the hundred stanzas of the Veda, to the praise of Rudra, bowing the while with their sincere souls unto Bhava of immeasurable power. Then those two terrible snakes, in consequence of the power of those adorations to Rudra, abandoned their snake-forms and assumed the forms of a foe-killing bow and arrow. Gratified (with what they saw), Krishna and Arjuna then seized that bow and arrow of great effulgence. And those high-souled heroes then brought them away and gave them unto the illustrious Mahadeva. Then from one of the sides of Siva’s body there came out a Brahmacharin of tawny eyes. And he seemed to be the refuge of asceticism. Of blue throat and red locks, he was endued with great might. Taking up that best of bows that Brahmacharin stood placing (both the bow and his feet properly). And fixing the arrow on the bowstring, he began to stretch the latter duly. Beholding the manner of his seizing the handle of the bow and drawing the string and placing of his feet, and hearing also the Mantras uttered by Bhava, the son of Pandu, of inconceivable prowess, learnt everything duly. The mighty and puissant Brahmacharin then sped that arrow to that same lake. And he once more threw that bow also in that self-same lake. Then Arjuna of good memory knowing that Bhava was gratified with him, and remembering also the boon the latter had given him in the forest, and the sight also he gave him of his person, mentally entertained the desire, “Let all this become productive of fruit!” Understanding this to be his wish, Bhava, gratified with him, gave him the boon. And the god also granted him the terrible Pasupata weapon and the accomplishment of his vow. Then having thus once more obtained the Pasupata weapon from the supreme god, the invincible Arjuna, with hair standing on end, regarded his business to be already achieved.