Śatapatha Brāhmaṇa ("of a hundred paths") 3.9.4.14

3.9.4.14

atha nigrābhyābhirupasṛjati | āpo ha vai vṛtraṃ jaghnustenaivaitadvīryeṇāpaḥ syandante tasmādenāḥ syandamānā na kiṃ cana pratighārayati tā ha svameva vaśaṃ ceruḥ kasmai nu vayaṃ tiṣṭhemahi yābhirasmābhirvṛtro hata iti sarvaṃ vā idamindrāya tasthānamāsa yadidaṃ kiṃ cāpi yo 'yam pavate

He then pours Nigrâbhyâ water on it. Now the waters, forsooth, slew Vritra and by virtue of that prowess of theirs they now flow. Wherefore nothing whatsoever can check them when they flow; for they followed their own free will, thinking, 'To whom, forsooth, should we submit (or stop), we by whom Vritra was slain!' Now all this (universe), whatsoever there is, had submitted to Indra, even he that blows yonder.