Śatapatha Brāhmaṇa ("of a hundred paths") 2.6.1.24

2.6.1.24

te sarva eva prācīnāvītino bhūtvā | etairvai havirbhiḥ pracariṣyanta itthādyajamānaśca brahmā ca purastātparītaḥ paścādagnīttadutāśrāvayantyoṃ3 svadhetyastu svadheti pratyāśrāvaṇaṃ svadhā nama iti vaṣaṭkāraḥ

They all shift their sacrificial cord over to the right shoulder, being now about to proceed with those (chief) oblations. The Sacrificer and Brahman, (being) thus (invested), step across (from the west) to the east side, and the Âgnîdhra (from the east) to the west side (of the fire). And furthermore, the (Adhvaryu's) call for the 'Sraushat' is 'Õm svadhâ!' and the (Âgnîdhra's) response is 'Astu svadhâ!' and the Vashat-call is 'Svadhâ namah!'