Śatapatha Brāhmaṇa ("of a hundred paths") 7.3.2.10

7.3.2.10

athāśvaṃ śuklam purastānnayanti | etadvai devā abibhayuryadvai na iha rakṣāṃsi nāṣṭrā na hanyuriti ta etaṃ vajramapaśyannamumevādityamasau vā āditya eṣo 'śvasta etena vajreṇa purastādrakṣāṃsi nāṣṭrā apahatyābhaye 'nāṣṭre svasti samāśnuvata tathaivaitadyajamāna etena vajreṇa purastādrakṣāṃsi nāṣṭrā apahatyābhaye 'nāṣṭre svasti samaśnuta āgaghantyagniṃ dakṣiṇataḥ pucasya citimupanidadhatyuttarato 'śvamākramayanti

And in front they lead a white horse. For at that time the gods were afraid lest the Rakshas, the fiends, should smite them here. They saw that thunderbolt, even yonder sun; for that horse is yonder sun: having driven off the Rakshas, the fiends, in front, by that thunderbolt, they obtained well-being in a place free from danger and devilry. They arrive at the (site of) the fire-altar; south of the tail (of the altar) they set down the layer (of bricks); from the north they make the horse step (on the site of the altar).