Śatapatha Brāhmaṇa ("of a hundred paths") 3.1.3.27

3.1.3.27

atha yatsvāhā svāheti karoti | yajño vai svāhākāro yajñamevaitadātmandhatte 'tro eva vācaṃ yacati vāgvai yajño yajñamevaitadātmandhatte

And when he calls, 'Hail! Hail!'--the 'Svâhâ' being the sacrifice--he thereby appropriates the sacrifice. Here now he restrains his speech; the sacrifice being speech: he thereby appropriates the sacrifice.