Śatapatha Brāhmaṇa ("of a hundred paths") 2.1.4.16

2.1.4.16

tato devā etaṃ vajraṃ dadṛśuḥ | yadaśvaṃ tam purastādudaśrayaṃstasyābhaye 'nāṣṭre nivāte 'gnirajāyata tasmādyatrāgnim manthiṣyantsyāttadaśvamānetavai brūyātsa pūrveṇopatiṣṭhate vajramevaitaducrayati tasyābhaye 'nāṣṭre nivāte 'gnirjāyate

The gods then perceived this thunderbolt, to wit, the horse. They made it stand before them, and in its safe and foeless shelter the fire was produced. For this reason let him (the Adhvaryu) direct (the Âgnîdhra) to lead the horse to where he is about to churn the fire. It stands in front of him: he thus raises the thunderbolt, and in its safe and foeless shelter the fire is produced.