Ramayana
यदि देवि प्रसन्ना मे रूपेणाप्रतिमा भुवि । मासं स्त्रीत्वमुपासित्वा मासं स्यां पुरुषः पुनः ॥ 7-87-26 ॥
"O goddess, if you are pleased with me, then let me, unequalled in beauty upon the earth, dwell in womanhood for a month and then again become a man for a month."
english translation