Mbh 7.81.1a

“Sanjaya said, ‘Then Partha, with a cheerful soul and joined hands and eyes expanded (in wonder), gazed at the god having the bull for his mark and who was the receptacle of every energy. And he beheld the offerings he made every night to Vasudeva lying by the side of the Three-eyed deity. The son of Pandu then, mentally worshipping both Krishna and Sarva, said unto the latter, “I desire (to obtain) the celestial weapon.” Hearing these words of Partha desiring the boon he sought, god Siva smilingly said unto Vasudeva and Arjuna, “Welcome to you, ye foremost of men! I know the wish cherished by you, and the business for which you have come here. I will give you what you wish. There is a celestial lake full of Amrita, not far from this place, ye slayers of foes! There were kept some time back, that celestial bow and arrow of mine. With them I slew in battle all the enemies of the gods. Bring hither, ye Krishna, that excellent bow with arrow fixed on it.” Hearing these words of Siva, Vasudeva with Arjuna answered, “So be it.” And then accompanied by all the attendants of Siva, those two heroes set out for that celestial lake which possessed hundreds of heavenly wonders, that sacred lake, capable of granting every object, which the god, having the bull for his mark, had indicated to them. And unto that lake, the Rishis Nara and Narayana (viz., Arjuna and Vasudeva) went fearlessly. And having reached that lake, bright as the disc of the sun, Arjuna and Achyuta beheld within its waters a terrible snake. And they beheld there another foremost of snakes, that had a thousand heads. And possessed of the effulgence of fire, that snake was vomiting fierce flames. Then Krishna and Partha having touched water, joined their hands, and approached those snakes, having bowed unto the god having the bull for his mark.