Shatapatha Brahmana 11.2.6.1

11.2.6.1

śiro ha vā etadyajñasya yatpraṇītāḥ sa yatpraṇītāḥ praṇayati śira evaitadyajñasya saṃskaroti sa vidyācira eva ma etatsaṃskriyata iti

The Pranîtâ water, doubtless, is the head of the sacrifice; and when he leads forward the Pranîtâ water, it is the head of the sacrifice he thereby forms, and he should know that it is that head of his own that is then being formed.