Śatapatha Brāhmaṇa ("of a hundred paths") 1.5.3.23

1.5.3.23

atha yathādevataṃ | svāhā devā ājyapā iti tatprayājānuyājāntsamasthāpayanprayājānuyājā vai devā ājyapā juṣāṇo agnirājyasya vetviti tadagniṃ sviṣṭakṛtaṃ samasthāpayannagnirhi sviṣṭakṛtsa eṣo 'vyetarhi tathaiva yajñaṃ saṃtiṣṭate yathaivainaṃ devāḥ samasthāpayaṃstasmāduttame prayāje svāhāsvāheti yajati yāvanti havīṃṣi bhavanti vijitamevaitadanu sarvaṃ yajñaṃ saṃsthāpayati tasmādyadata ūrdhvaṃ viloma yajñe kriyeta na tadādriyeta saṃsthito me yajña iti ha vidyātsa haiṣa yajño yātayāmevāsa yathā vaṣaṭkṛtaṃ hutaṃ svāhākṛtaṃ

And so with the (other) deities respectively. With 'Svâhâ the butter-drinking gods!' they established the fore-offerings and the after-offerings (anuyâgas), for the fore-offerings and after-offerings, doubtless, represent the butter-drinking gods. With the formula 'May Agni graciously accept of the butter!' they established Agni as Svishtakrit ('maker of good offering'), for Agni is indeed the maker of good offering. And till this day that sacrifice stands as firm as the gods established it. This is the reason why at the last fore-offering he prays with Svâhâ! Svâhâ!' according to the number of oblations (there are at the chief sacrifice). After he (the sacrificer) has won his battle, he establishes the entire sacrifice on a firm basis, so that, if after this he should violate the proper order of the sacrifice, he need not heed it; for he will know that his sacrifice is firmly established. Now what with exclaiming 'Vashat,' with offering, and with calling out 'Svâhâ,' this same sacrifice was well-nigh exhausted.