Shatapatha Brahmana 9.1.1.42

9.1.1.42

atha tadarkaparṇaṃ cātvāle prāsyati | etadvā enenaitadraudraṃ karma karoti tadetadaśāntaṃ tadetattiraḥ karoti nedidamaśāntaṃ kaścidabhitiṣṭhāttannenddhinasaditi tasmāccātvāle yadveva cātvāle 'gnireṣa yaccātvālastatho hainadeṣo 'gniḥ saṃdahatyathātaḥ sampadeva

He then throws that arka-leaf into the pit; for it is therewith that he performs that sacrificial work sacred to Rudra, and that same (leaf) is inauspicious; he now puts it away, lest any one should tread on this inauspicious (leaf), and suffer injury thereby: hence (he throws it) into the pit. And, again, as to why (he throws it) into the pit,--the pit, doubtless, means the fire, and thus that fire consumes it. Now as to the (symbolic) correspondence.