Śatapatha Brāhmaṇa ("of a hundred paths") 14.3.1.2

14.3.1.2

athāgnīdhraḥ āhavanīye trīñcālākānupakalpayate teṣāmekamujjvalayya mukhadaghne dhārayamāṇo juhoti yajñasya śīrṣacinnasya śugudakrāmatsemāṃlokānāviśattayaivainametacucā samardhayati kṛtsnaṃ karoti

The Âgnîdhra then brings three bundles of faggots to the Âhavanîya, and kindling one of them, he offers (thereon) whilst holding it on a level with (the Sacrificer's) mouth. When the sacrifice had its head cut off its heat went out of it, and entered these worlds: it is with that heat he thus supplies and completes it.