Śatapatha Brāhmaṇa ("of a hundred paths") 8.6.2.19

8.6.2.19

tadete sāmanidhane abhyukte | arko devānām parame vyomannarkasya devāḥ parame vyomannityetadvai devānāṃ viśatām prajāpatiruttamo 'viśattasmādāhārko devānām parame vyomannityatha yadāhārkasya devāḥ parame vyomannityayaṃ vā agnirarkastasyaitaduttamāyāṃ citau sarve devā viṣṭāstasmādāhārkasya devāḥ parame vyomanniti

It is with reference to this that these two sâma-nidhanas (finales of sâman-hymns) are uttered,--'The light (is) in the highest heaven of the gods,' and, 'The gods (are) in the highest heaven of the light;'--for when on that occasion the gods were entering (heaven), Pragâpati was the last to enter: that is why he says, 'The light (is) in the highest heaven of the gods.' And as to why he says, 'The gods (are) in the highest heaven of light,'--the light, doubtless, is this Agni (the fire-altar), and it is on his highest layer that all the gods have thus entered: this is why he says, 'The gods are in the highest heaven of light.'