Brihadaranyaka Upanishad 1.4.3

1.4.3

sa vai naiva reme tasmādekākī na ramate sa dvitīyamaicchat | sahaitāvānāsa yathā strīpumām̐sau saṃpariṣvaktau sa imamevātmānaṃ dvedhāpātayattataḥ patiśca patnī cābhavatāṃ tasmādidamardhabṛgalamiva sva iti ha smāha yājñavalkyastasmādayamākāśaḥ striyā pūryata eva tām̐ samabhavattato manuṣyā ajāyanta

He was not at all happy. Therefore people (still) are not happy when alone. He desired a mate. He became as big as man and wife embracing each other. He parted this body into two. From that came husband and wife. Therefore, said Yâjñavalkya, this (body) is one-half of oneself, like one of the two halves of a split pea. Therefore this space is indeed filled by the wife. He was united with her. From that men were born.