Agniveshyagrihyasutra 2.7.2.12

2.7.2.12

pariśrite uddhatyāvokṣya sikatopopta udumbaraśākhābhiḥ plakṣaśākhābhiḥ pālāśaśākhābhirvā pracchādya yājñikāt kāṣṭhādagniṃ mathitvā śrotriyāgārādvāhṛtya sate kṛtvā prajvalayitvā śākhām apohya oṃ bhūrbhuvaḥsvaroṃ pratiṣṭha iti nyupyopasamādadhāti \

In an enclosed place, having raised [the ground] and sprinkled [it], sand having been scattered, having covered [it] with udumbara branches, plakṣa branches, or palāśa branches, having churned the fire from sacrificial wood or brought [it] from the house of a learned Brahmin, having placed [it] in a vessel, having kindled [it], having cast aside the branch, he sets [it] down and lays [it] on, saying: "Oṃ bhūrbhuvaḥsvaḥ oṃ, be established!"