Shatapatha Brahmana 6.5.3.8

6.5.3.8

athaināṃ dhūpayati | sthemne nvevātho karmaṇaḥ prakṛtatāyai yadveva dhūpayati śira etadyajñasya yadukhā prāṇo dhūmaḥ śīrṣaṃstatprāṇaṃ dadhāti

He (the Adhvaryu) now fumigates it (the fire-pan)--just for the sake of strength, or to (mark) the progress of the work. And, again, as to why he fumigates,--that fire-pan is the head of the sacrifice, and the smoke its breath: he thus puts breath into the head.