Śatapatha Brāhmaṇa ("of a hundred paths") 3.9.4.6

3.9.4.6

atha vācaṃ yacati | devā ha vai yajñaṃ tanvānāste 'surarakṣasebhya āsaṅgādbibhayāṃ cakruste hocurupāṃśu yajāma vācaṃ yacāmeti ta upāṃśvayajanvācamayacan

Thereupon he restrains-speech. For once on a time, the gods, while performing sacrifice, were afraid of an attack from the Asura-Rakshas. They said, 'Let us sacrifice in a low voice, let us restrain speech!' They sacrificed (with formulas muttered) in a low voice and restrained speech.