Śatapatha Brāhmaṇa ("of a hundred paths") 2.4.2.14

2.4.2.14

atha parastādulmukaṃ nidadhāti | sa yadanidhāyolmukamathaitatpitṛbhyo dadyādasurarakṣasāni haiṣāmetadvimathnīraṃstatho haitatpitṝṇāmasurarakṣasāni na vimathnate tasmātparastādulmukaṃ nidadhāti

He then lays down a firebrand at the farther (south) end (of the line). For were he to present that (food) to the fathers, without having laid down a firebrand, the Asuras and Rakshas would certainly tamper with it. And thus the Asuras and Rakshas do not tamper with that (food) of the fathers: this is why he lays down the firebrand at the farther end (of the line).