Śatapatha Brāhmaṇa ("of a hundred paths") 6.1.3.1

6.1.3.1

prajāpatirvā idamagra āsīt | eka eva so 'kāmayata syām prajāyeyeti so 'śrāmyatsa tapo 'tapyata tasmācrāntāttepānādāpo 'sṛjyanta tasmātpuruṣāttaptādāpo jāyante

Verily, Pragâpati alone was here in the beginning. He desired, 'May I exist, may I reproduce myself!' He toiled, he practised austerity (or, became heated). From him, worn out and heated, the waters were created: from that heated Person the waters are born.