Śatapatha Brāhmaṇa ("of a hundred paths") 3.6.1.29

3.6.1.29

tāndevāḥ pratisamaindhata | yathā pratyavasyettasmādenāntsavane savana eva pratisamindhate tasmādyaḥ samṛddhaḥ sa āgnīdhraṃ kuryādyo vai jñāto 'nūcānaḥ sa samṛddhastasmādagnīdhe prathamāya dakṣiṇāṃ nayantyato hi viśve devā amṛtatvamapājayaṃstasmādyaṃ dīkṣitānāmabalyaṃ vindedāgnīdhramenaṃ nayateti brūyāttadanārtaṃ tannāriṣyatīti tadyadato viśve devā amṛtatvamapājayaṃstasmādvaiśvadevam

The gods kindled them again, as one would (light the fire where he is going to) stay. Therefore they are kindled at every Soma feast. Wherefore the duties of the Agnîdh should be discharged by one who is accomplished. Now he who is known and learned in sacred lore is truly accomplished: hence they take to the Agnîdh his Dakshinâ first, since it is from thence (from the Agnîdh's fire) that the gods gained immortality. And if weakness were to come upon one of those that are consecrated, let (the Adhvaryu) say, 'Lead him to the Âgnîdhra!'--thinking 'that is unscathed, there he will not meet with affliction.' And because the All-gods gained immortality from there, therefore it is sacred to the All-gods.