Śatapatha Brāhmaṇa ("of a hundred paths") 7.3.2.18

7.3.2.18

athaitaṃ sāye bhūte 'śvam pariṇayanti | etadva devā abibhayuryadvai na imamiha rakṣāṃsi nāṣṭrā na hanyuriti tasmā etaṃ vajramabhigoptāramakurvannamumevādityamasau vā āditya eṣo 'śvastathaivāsmā ayametaṃ vajramabhigoptāraṃ karoti

Now that horse they lead about when evening is closing in; for at that time the gods were afraid lest the Rakshas, the fiends, should there smite that (Agni, or altar) of theirs. They made that thunderbolt, to wit, yonder sun, his protector, for that horse is yonder sun; and in like manner does this one now make that thunderbolt his (Agni's) protector.