Mbh 3.273.1b

And seized with jealousy, they resolved upon performing penances. And with ascetic penances of the most severe kind, they gratified Brahma. And the Ten-headed Ravana, supporting life by means of air alone and surrounded by the five sacred fires and absorbed in meditation, remained standing on one leg for a thousand years. And Kumvakarna with head downwards, and with restricted diet, was constant in austerities. And the wise and magnanimous Vibhishana, observing fasts and subsisting only on dry leaves and engaged in meditation, practised severe austerities for a long period. And Khara and Surpanakha, with cheerful hearts, protected and attended on them while they were performing those austerities. And at the close of a thousand years, the invincible Ten-headed One, cutting off his own heads, offered them as offering to the sacred fire. And at this act of his, the Lord of the Universe was pleased with him. And then Brahma, personally appearing to them, bade them desist from those austerities and promised to grant boons unto every one of them. And the adorable Brahma said, I am pleased with you, my sons! Cease now from these austerities and ask boons of me! Whatever your desires may be, they, with the single exception of that of immortality, will be fulfilled! As thou hast offered thy heads to the fire from great ambition, they will again adorn thy body as before, according to thy desire. And thy body will not be disfigured and thou shall be able to assume any form according to thy desire and become the conqueror of thy foes in battle. There is no doubt of this!’ thereupon Ravana said, ‘May I never experience defeat at the hands of Gandharvas, Celestials, Kinnaras, Asuras, Yakshas, Rakshasas, Serpents and all other creatures!’ Brahma said, ‘From those that hast named, thou shalt never have cause of fear; except from men (thou shalt have no occasion for fear). Good betide thee! So hath it been ordained by me!’