Shatapatha Brahmana 6.7.1.5

6.7.1.5

yadeva rukbham pratimucya bibharti | etadvai devā abibhayuryadvai na imamiha rakṣāṃsi nāṣṭrā na hanyuriti tasmā etamantikādgoptāramakurvannamumevādityamasau vā āditya eṣa rukbhastathaivāsmā ayametamantikādgoptāraṃ karoti

And, again, why he puts on and wears the gold plate;--the gods now were afraid lest the Rakshas, the fiends, should destroy here that (Agni) of theirs. They made that (plate), yonder sun, to be his (Agni's) protector (standing) by his side, for the gold plate is yonder sun: and in like manner does this (Sacrificer) now make that (plate) to be his (Agni's) protector by his side.