Śatapatha Brāhmaṇa ("of a hundred paths") 1.4.1.26

1.4.1.26

bṛhacocā yaviṣṭyeti | bṛhadu hyeṣa ṣocati samiddho yaviṣṭyeti yaviṣṭho hyagnistasmādāha yaviṣṭhyeti saiṣitameva lokamabhyanūktāntarikṣalokameva tasmādāgneyī satyaniruktānirukto hyeṣa loka etamevaitayā lokaṃ jayati yasyaivaṃ viduṣa etāmanvāhuḥ

'Shine forth, O youngest, brilliantly!' he adds; for brilliantly he shines, when kindled; and 'O youngest!' he says, because he is really the youngest Agni: therefore he says, 'O youngest!' This (verse) is recited with reference to yonder world, to wit, the aërial world; hence this (verse), which is addressed to Agni, is undefined, for undefined is yonder world: that world he thereby gains, for whom, knowing this, they recite this verse.