Śatapatha Brāhmaṇa ("of a hundred paths") 4.6.8.3

4.6.8.3

atha dīkṣiṣyamāṇāḥ samavasyanti | te yadyagniṃ ceṣyamāṇā bhavantyaraṇiṣvevāgnīntsamārohyopasamāyanti yatra prājāpatyena paśunā yakṣyamāṇā bhavanti mathitvopasamādhāyoddhṛtyāhavanīyaṃ yajanta etena prājāpatyena paśunā

Now those who are about to consecrate themselves settle (the time and place) between them. If they intend to construct a fire-altar, they take up their (ordinary sacrificial) fires on churning-sticks and betake themselves together to where they are about to perform the animal offering to Pragâpati. Having churned (the fire), and put fire-wood on, they take out the Âhavanîya fire, and perform that animal offering to Pragâpati.