Shatapatha Brahmana 11.1.6.1

11.1.6.1

āpo ha vā idamagre salilamevāsa tā akāmayanta kathaṃ nu prajāyemahīti tā aśrāmyaṃstāstapo'tapyanta tāsu tapastapyamānāsu hiraṇmayamāṇḍaṃ sambabhūvājāto ha tarhi saṃvatsara āsa tadidaṃ hiraṇmayamāṇḍaṃ yāvatsaṃvatsarasya velā tāvatparyaplavata

Verily, in the beginning this (universe) was water, nothing but a sea of water. The waters desired, 'How can we be reproduced?' They toiled and performed fervid devotions, when they were becoming heated, a golden egg was produced. The year, indeed, was not then in existence: this golden egg floated about for as long as the space of a year.