Shatapatha Brahmana 7.5.1.11

7.5.1.11

tribhirupadadhāti | traya ime lokā atho trivṛdagniryāvānagniryāvatyasya mātrā tāvataivainametadupadadhāti tribhirabhyanakti tatṣaṭtasyokto bandhuravakā adhastādbhavantyavakā upariṣṭādāpo vā avakā apāmevainametanmadhyato dadhāti sādayitvā sūdadohasādhivadati tasyokto bandhuḥ

With three (formulas) he puts it on (the altar);--three are these worlds, and threefold is Agni: as great as Agni is, as great as is his measure, by so much he thus puts it on. With three (formulas) he anoints it; that makes six: the significance of this (number) has been explained. There are avakâ-plants below and avakâ-plants above (the tortoise),--the avakâ-plant means water: he thus places it in the midst of water. Having 'settled' it, he pronounces the Sûdadohas upon it: the significance of this has been explained.