Shatapatha Brahmana 4.5.1.8

4.5.1.8

tadyanmaitrāvaruṇī vaśā bhavati | yatra vai devā retaḥ siktam prājanayaṃstadāgnimārutamityukthaṃ tasmiṃstadvyākhyāyate yathā taddevā retaḥ prājanayaṃstato 'ṅgārāḥ samabhavannaṅgārebhyo 'ṅgirasastadanvanye paśavaḥ

And again, why there is a barren cow for Mitra and Varuna. Now, when the gods caused the cast seed to spring,--there is that sastra called Âgnimâruta: in connection therewith it is explained how the gods caused that seed to spring. From it the coals (aṅgâra) sprung, and from the coals the Aṅgiras; and after that the other animals.