Śatapatha Brāhmaṇa ("of a hundred paths") 11.3.1.8

11.3.1.8

yo jāgāra bhuvaneṣu viśvā jātāni yo'bibhaḥ tasmintsāsya hutāhutirgṛhe yāmasya juhvatīti prāṇamevaitadāha tasmādāhuḥ prāṇa evāgnihotramiti

'He who waketh in the worlds and sustaineth all beings, in him that offering of his is offered, (and in him) do they, at his house, perform the offering of the progress;'--he thereby means the breath; whence they say, 'The Agnihotra is breath.'