Śatapatha Brāhmaṇa ("of a hundred paths") 2.1.4.25

2.1.4.25

sa vai tūṣṇīmevāgra upaspṛśati | athodyacatyathopaspṛśati bhūrbhuvaḥ svarityeva tṛtīyenādadhāti trayo vā ime lokāstadimānevaitallokānāpnotyetannvekam

In the first place he silently touches (the footprint with the burning fire-wood). He then lifts it up and touches once more with it; and at the third time he lays it down with (Vâg. S. III, 5), 'Earth! ether! heaven!' For there are three worlds indeed; so that he thereby obtains these (three) worlds. This now is one (mode of laying down the fire).