Shatapatha Brahmana 11.1.6.16

11.1.6.16

tā arcantyaḥ śrāmyantyaśceruḥ tata etam parameṣṭhī prājāpatyo yajñamapaśyadyaddarśapūrṇamāsau tābhyāmayajata tābhyāmiṣṭvākāmayatāhamevedaṃ sarvaṃ syāmiti sa āpo'bhavadāpo vā idaṃ sarvaṃ tā yatparame sthāne tiṣṭhanti yo hīhābhikhanedapa evābhivindetparamādvā etatsthānādvarṣati yaddivastasmātparameṣṭhī nāma

They went on singing praises and toiling. Then Parameshthin, son of Pragâpati, saw that sacrifice, the New and Full-moon offerings, and performed these offerings. Having performed them, he desired, 'Would I were everything here!' He became the waters, for the waters are everything here, inasmuch as they abide in the furthest place; for he who digs here on earth finds indeed water; and, in truth, it is from that furthest place, to wit, from yonder sky that he rains, whence the name Parameshthin (abiding in the furthest, highest place).