Śatapatha Brāhmaṇa ("of a hundred paths") 4.3.2.13

4.3.2.13

taddhaike | othāmodaiva vāgiti pratigṛṇanti vākpratigara etadvācamupāpnuma iti vadantastadu tathā na kuryādyathā vai kathā ca pratigṛṇātyupāptaivāsya vāgbhavati vācā hi pratigṛṇāti tasmādomityeva pratigṛṇīyāttaddhi satyaṃ taddevā viduḥ

Now some respond with 'Othâmo daiva vâk,' saying, 'The response is speech (vâk): thus we obtain speech.' But let him not do this; for surely, in whichsoever way he may respond, speech is obtained by him, since he responds by speech. Let him therefore respond with 'Om!' only, for that is truth, that the gods know.