Śatapatha Brāhmaṇa ("of a hundred paths") 3.5.2.14

3.5.2.14

atha paridhīnparidadhāti | dhruvo 'si pṛthivīṃ dṛṃheti madhyamaṃ dhruvakṣidasyantarikṣaṃ dṛṃheti dakṣiṇamacyutakṣidasi divaṃ dṛṃhetyuttaramagneḥ purīṣamasīti sambhārānupanivapati tadyatsambhārā bhavantyagnereva sarvatvāya

He then lays the enclosing-sticks round (the navel); with the text (Vâg. S. V, 13), 'Thou art firm, steady thou the earth!' the middle one; with, 'Thou art firmly settled, steady thou the air!' the right one; with, 'Thou art immovably settled, steady thou the sky!' the left one. With, 'Thou art Agni's provision' he throws the 'equipments' upon (the high altar). Wherefore are the equipments?--for Agni's completeness.