Śatapatha Brāhmaṇa ("of a hundred paths") 1.4.4.14

1.4.4.14

atha sammārṣṭi | yunaktyevainametadyukto devebhyo yajñaṃ vahāditi tasmātsammārṣṭi parikrāmaṃ sammārṣṭi parikrāmaṃ hi yogyaṃ yuñjanti tristriḥ sammārṣṭi trivṛddhi yajñaḥ

He (the Âgnîdhra) then sweeps (the fire with the band of the fire-wood): he thereby harnesses it, thinking, 'Now that it has been harnessed, may it convey the sacrifice to the gods!' for this reason he sweeps it. While sweeping it he moves around, since in harnessing they move around the team. He sweeps thrice each time (i.e. thrice along each of the three enclosing-sticks): threefold is the sacrifice.