Śatapatha Brāhmaṇa ("of a hundred paths") 3.9.2.9

3.9.2.9

yadyu etadubhayaṃ na vindet | ulkuṣīmevādāyopapareyāttāmuparyupari dhārayangṛhṇīyāddhiraṇyaṃ voparyupari dhārayangṛhṇīyāttadetasya rūpaṃ kriyate ya eṣa tapati

But if he cannot obtain either kind (of water), let him seize a firebrand and betake himself (to the flowing water), and let him take thereof while holding that (firebrand) close above it; or let him take it while holding a piece of gold close above it: thus it is made like yonder burning (sun).