Shatapatha Brahmana 4.6.9.21

4.6.9.21

audumbarīmupasaṃsṛpya vācaṃ yacanti | viduhanti vā ete yajñaṃ nirdhayanti ye vācā yajñaṃ tanvate vāgghi yajñastāmeṣām puraikaika eva vācaṃyama āste vācamāpyāyayaṃstayāpīnayāyātayāmnyottaramahastanvate 'thātra sarvaiva vāgāptā bhavatyapavṛktā tāṃ sarva eva vācaṃyamā vācamāpyāyayanti tayāpīnayāyātayāmnyātirātraṃ tanvate

Having 'crept' up to the Udumbara post, they restrain their speech. Now, they who perform the sacrifice with speech, milk and suck out the sacrifice; for sacrifice is speech. And previously to this, each of them sits speechless, strengthening his speech, and with their speech thus strengthened and reinvigorated they perform the last day. But at this (disputation) the entire speech, thus obtained, becomes exhausted: that speech they all strengthen (by remaining) speechless, and with it thus strengthened and reinvigorated they perform the Atirâtra.