Mbh 1.225.4c

And the god spake unto the monarch in a calm and grave voice, saying, ‘O tiger among kings, O chastiser of foes, I have been gratified with thee for thy asceticism! Blest be thou! Ask now the boon that thou, O king, desirest.’ Hearing these words of Rudra of immeasurable energy, the royal sage bowed unto that deity and replied, saying, ‘O illustrious one, O thou that art worshipped by the three worlds, if thou hast been gratified with me, then, O god of gods, assist me thyself, O lord of the celestials, in my sacrifice!’ Hearing these words spoken by the monarch, the illustrious god was gratified, and smilingly said, ‘We do not ourselves assist at sacrifices: but as thou, O king, hast undergone severe penances, desirous of obtaining a boon, I will, O chastiser of foes, assist at thy sacrifice, upon, O king, this condition.’ And Rudra continued, ‘If, O king of kings, thou canst, for twelve years, pour without intermission libations of clarified butter into the fire, thyself leading all the while the life of a Brahmacharin with rapt attention, then thou shalt obtain from me what thou askest.’ King Swetaki, thus addressed by Rudra, did all that he was directed to do by the wielder of the trident. And after twelve years had elapsed, he again came unto Maheswara. And Sankara, the Creator of the worlds upon seeing Swetaki, that excellent monarch, immediately said, in great gratification, ‘I have been gratified by thee, O best of kings, with this thy own act! But, O chastiser of foes, the duty of assisting at sacrifices properly belongeth to Brahmanas. Therefore, O oppressor of foes, I will not myself assist at thy sacrifice today. There is on earth an exalted Brahmana who is even a portion of my own self. He is known by the name of Durvasa. Even that Brahmana endued with great energy will assist you in thy sacrifice. Let, therefore, every preparation be made.’