Śatapatha Brāhmaṇa ("of a hundred paths") 2.6.2.15

2.6.2.15

atha punaḥ prasalavi triḥ pariyanti | dakṣiṇānūrūnupāghnānā etenaiva mantreṇa tadyatpunaḥ prasalavi triḥ pariyanti prasalavi na idaṃ karmānusaṃtiṣṭhātā iti tasmātpunaḥ prasalavi triḥ pariyanti

Then they (the Sacrificer and priests) again walk round thrice sunwise, beating their right thighs, with the same text. As to why they again walk round thrice sunwise,--they think, 'Sunwise this sacred work of ours shall be accomplished,' and therefore they again walk thrice round sunwise.