Śatapatha Brāhmaṇa ("of a hundred paths") 3.2.2.7

3.2.2.7

anenaiva vācaṃ visarjayet | vrataṃ kṛṇuta vrataṃ kṛṇutāgnirbrahmāgniryajño vanaspatiryajñiya ityeṣa hyasyātra yajño bhavatyetaddhaviryathā purāgnihotraṃ tadyajñenaivaitadyajñaṃ sambhṛtya yajñe yajñam pratiṣṭhāpayati yajñena yajñaṃ saṃtanoti saṃtataṃ hyevāsyaitadvratam bhavatyā sutyāyai triṣkṛtva āha trivṛddhi yajñaḥ

Let him rather break silence with this one (Vâg. S. IV, 11), 'Prepare ye the fast-food! prepare ye the fast-food! [prepare ye the fast-food!] Agni is the Brahman, Agni is the sacrifice; the tree is meet for the sacrifice.' For this indeed is his sacrifice, this is his havis-offering at this (rite of consecration), even as the Agnihotra was heretofore. In thus preparing the (Soma) sacrifice by means of the sacrifice, he establishes the sacrifice in the sacrifice, and carries on the sacrifice by means of the sacrifice; for (the partaking of) that fast-food is indeed carried on by him till the pressing of the Soma. Thrice he pronounces (the first words), for threefold is the sacrifice.