Shatapatha Brahmana 14.1.3.15

14.1.3.15

sa yatraitāṃ hotānvāha saṃsīdasva mahām mahām asīti tadubhayata ādīptā mauñjāḥ pralavā bhavanti tānupāsya teṣu pravṛṇakti yajñasya śīrṣacinnasya raso vyakṣarattata etā oṣadhayo jajñire tenaivainametadrasena samardhayati kṛtsnaṃ karoti

And when the Hotri recites this (verse, Rig-v. I, 36, 9), 'Sit thee down: thou art great...,' sheaths of reed-grass are kindled on both sides, and throwing them (on the mound), he puts (the Mahâvîra pot) thereon. When the sacrifice had its head cut off, its life-sap flowed away, and therefrom these plants grew up: with that life-sap he thus supplies and completes it.