Shatapatha Brahmana 10.2.5.8

10.2.5.8

atha yadi tisraḥ trayo vā ime lokā ime lokā upasada ādityaḥ pravargyo'muṃ tadādityameṣu lokeṣu pratiṣṭhāpayati tasmādeṣa eṣu lokeṣu pratiṣṭhitaḥ

And if there are three (Upasad-days)--there being these three worlds--the Upasads are these three worlds, and the Pravargya is the sun: he thus establishes yonder sun in these worlds, whence he is established in these worlds.