Agniveshyagrihyasutra 2.4.9.5

2.4.9.5

brāhmaṇānāhūya prāgekaḥ sthitvā pratyag dvau sthitvā dakṣiṇahastaṃ parivrājena bhūrbhuvaḥsvaḥ iti sūtraṃ kṛtvā triguṇīkṛtya yājñike vṛkṣe visṛjati \

Having called Brahmins, one standing to the east, two standing to the west, twisting the cord to the right with "Bhūr bhuvaḥ svaḥ," making it threefold, he fastens it onto a sacrificial tree.