Śatapatha Brāhmaṇa ("of a hundred paths") 7.1.2.18

7.1.2.18

athaite dve yajuṣī | so anuṣṭubeva vāgvā anuṣṭuptadyadidaṃ dvayaṃ vāco rūpaṃ daivaṃ ca mānuṣaṃ coccaiśca śanaiśca tadete dve

Then there are these two formulas, and they are indeed an anushtubh--the Anushtubh is speech: thus what twofold form of speech there is, the divine and the human, loud and low, that is those two.