Śatapatha Brāhmaṇa ("of a hundred paths") 2.1.1.11

2.1.1.11

tatho evaiṣa etat | imām pratiṣṭhāṃ śarkarābhiḥ paribṛṃhate tasyāṃ dhruvāyāmaśithilāyāmagnī ādhatte tato 'syai sapatnānnirbhajati tasmācarkarāḥ sambharati

And in like manner that one (the Adhvaryu) now fastens down that resting-place by means of pebbles; and on it, when firm and steady, he sets up the two fires; whereupon he excludes the (sacrificer's) enemies from any share in it. This is the reason why he brings pebbles.