Śatapatha Brāhmaṇa ("of a hundred paths") 14.2.2.43

14.2.2.43

atha dakṣiṇataḥ sikatā upakīrṇā bhavanti tanmārjayante ya eva mārjālīye bandhuḥ so'trānupraharati śākalānathopasadā carantyetadu yajñasya śiraḥ saṃskṛtaṃ yathā-yathainaṃ tadaśvinau pratyadhattām

Now, on the south side sand has been strewn; there they cleanse themselves: in this there is the same significance as in the Mârgâlîya. The pieces of wood he throws into the fire. They then proceed with the Upasad. And thus the head of the sacrifice has been set right in the very same manner in which the Asvins then restored it.