Śatapatha Brāhmaṇa ("of a hundred paths") 14.2.2.50

14.2.2.50

tadāhuḥ yatpraṇītāḥ praṇayanti yajñe'tha kasmādatra na praṇayatīti śiro vā etadyajñāsya yatpraṇītāḥ śiraḥ pravargyo necirasā śiro'bhyārohayāṇīti

And, again, they say, 'Seeing that at the sacrifice they lead forward the Pranîtâ (water), wherefore do they not lead it forward on this occasion?' Well, this--to wit, the Pranîtâ (water)--being the head of the sacrifice, and the Pravargya also being its head, (he does so) thinking, 'Lest I cause the head to be overtopped by a head.'