Śatapatha Brāhmaṇa ("of a hundred paths") 3.7.4.7

3.7.4.7

athāhāgnaye samidhyamānāyānubrūhīti | sa uttaramāghāramāghāryāsaṃsparśayantsrucau paryetya juhvā paśuṃ samanakti śiro vai yajñasyottara āghāra eṣa vā atra yajño bhavati yatpaśustadyajña evaitaciraḥ pratidadhāti tasmājjuhvā paśuṃ samanakti

Thereupon he says (to the Hotri), 'Recite to the fire being kindled!' when he has made the second libation of ghee, and returned (to his former place) without letting the two spoons touch one another, he anoints the victim with the (ghee in the) guhû, For the second libation is the head of the sacrifice, and the sacrifice here indeed is that victim: hence he thereby puts the head on the sacrifice and therefore anoints the victim with the guhû.