Śatapatha Brāhmaṇa ("of a hundred paths") 1.4.4.15

1.4.4.15

sa sammārṣṭi | agne vājajidvājaṃ tvā sariṣyantaṃ tvā vājajitaṃ sammārjmīti yajñaṃ tvā vakṣyantaṃ yajñiyaṃ sammārjmītyevaitadāhāthopariṣṭāttūṣṇīṃ tristadyathā yuktvā prājetprehi vahetyevamevaitatkaśayopakṣipati prehi devebhyo yajñaṃ vaheti tasmādupariṣṭāttūṣṇīṃ tristadyadetadantareṇa karma kriyate tasmādidam manaśca vākca samānameva sannāneva

He sweeps (once), with the text (Vâg. S. II, 7 a), 'O Agni, food-gainer! I cleanse thee, the food-gainer, who art about to hasten to the food!' Whereby he says, 'I cleanse thee who art going to convey the sacrifice (to the gods), thee fit for the sacrifice!' He then sweeps thrice over (the fire) silently: for just as, after harnessing (the animal), one urges it on, saying, 'Go on! pull!' so does he thereby strike it with the lash, thinking, 'Go on and convey the sacrifice to the gods!' That is why he (sweeps) thrice over it silently; and in like manner as this act is performed between (the two sprinklings of butter), so this mind and speech, though forming one, thereby become, as it were, separate.