Śatapatha Brāhmaṇa ("of a hundred paths") 7.4.1.6

7.4.1.6

tadyattatsatyam | āpa eva tadāpo hi vai satyaṃ tasmādyenāpo yanti tatsatyasya rūpamityāhurapa eva tasya sarvasyāgramakurvaṃstasmādyadaivāpo yantyathedaṃ sarvaṃ jāyate yadidaṃ kiṃ ca

Now that truth is the same as the waters, for the waters are the truth. Hence they say, 'Whereby the waters flow, that is a form of the truth.' It is the waters indeed that were made first of this universe: hence when the waters (rains) flow, then everything whatsoever exists is produced here.