Śatapatha Brāhmaṇa ("of a hundred paths") 14.2.1.11

14.2.1.11

atha pinvane pinvayati aśvibhyām pinvasvetyaśvināvevaitadāhāśvinau vā etadyajñasya śiraḥ pratyadhattāṃ tāvevaitatprīṇāti tasmādāhāśvibhyām pinvasveti

He then causes it to flow into the milking-bowl, with (Vâg. S. XX XVIII, 4), 'Flow for the Asvins!'--with regard to the Asvins he thus says this, for it was the Asvins who restored the head of the sacrifice: it is them he thus pleases, and therefore he says, 'Flow for the Asvins!'
12, 'Flow for Sarasvatî!'--Sarasvatî, doubtless, is Speech, and with speech the Asvins then restored the head of the Sacrifice: it is those (Asvins) he thus pleases, and therefore he says, 'Flow for Sarasvatî!'