Śatapatha Brāhmaṇa ("of a hundred paths") 10.4.1.7

10.4.1.7

tāvetāvindrāgnī eva cinvanti yaddhi kiṃ caiṣṭakamagnireva tattasmāttadagninā pacanti yaddhi kiṃ cāgninā pacantyagnireva tadatha yatpurīṣaṃ sa indrastasmāttadagninā na pacanti nedagnirevāsannendra iti tasmādetāvindrāgnī eva citau

It is these two, Indra and Agni, that they build up. Whatever is of brick that is Agni: whence they bake that (part) by fire, and all that is baked by fire is Agni. And what filling of earth there is (in the altar) that. is Indra: whence they do not bake that (part) by fire, lest it should be Agni, and not Indra. Thus it is these two, Indra and Agni, that are built up.