Śatapatha Brāhmaṇa ("of a hundred paths") 6.3.1.18

6.3.1.18

yasya prayāṇamanvanya idyayuriti | prajāpatirvā etadagre karmākarottattato devā akurvandevā devasya mahimānamojaseti yajño vai mahimā devā devasya yajñaṃ vīryamojasetyetadyaḥ pārthivāni vimame sa etaśa iti yadvai kiṃ cāsyāṃ tatpārthivaṃ tadeṣa sarva vimimīte raśmibhirhyenadabhyavatanoti rajāṃsi devaḥ savitā mahitvanetīme vai lokā rajāṃsyasāvādityo devaḥ savitā tāneṣa mahimnā vimimīte

[Vâg. S. XI, 6; Rik S. V, 81, 3] 'Whose course the others have followed,'--for Pragâpati first performed this rite, whereupon the gods performed it;--'the gods with vigour, the god's greatness,'--the greatness is the sacrifice, thus: 'the gods with vigour (followed) the god's sacrifice, his energy;'--'that dappled steed who hath measured the terrestrial (regions),'--whatsoever is on this earth that is terrestrial, all that he measures out; for with his rays he reaches down to it;--'the regions, he the god Savitri by his greatness,'--the regions are these worlds, and the god Savitri is yonder sun: he measures them by his greatness.