Śatapatha Brāhmaṇa ("of a hundred paths") 8.2.1.4

8.2.1.4

sa upadadhāti | dhruvakṣitirdhruvayonirdhruvāsīti yadvai sthiraṃ yatpratiṣṭhitaṃ taddhruvamatha vā eṣāmeṣa loko 'dhruva ivāpratiṣṭhita iva manasyāsīttamevaitatsthiraṃ dhruvaṃ kṛtvā pratyadhattāṃ dhruvaṃ yonimāsīda sādhuyeti sthiraṃ yonimāsīda sādhuyetyetadukhyasya ketum prathamaṃ juṣāṇetyayaṃ vā agnirukhyastasyaiṣa prathamaḥ keturyatprathamā citistaṃ juṣāṇetyetadaśvinādhvaryū sādayatāmiha tvetyaśvinau hyadhvaryū upādhattām

He lays down (the first Âsvinî brick, with Vâg. S. XIV, 1), 'Thou art firmly-founded, firmly-seated, firm!' for what is steady and settled, that is firm. Now that world was to their minds, as it were, unfirm and unsettled: having thereby made it firm, steady, they (the Asvins) went on laying down (bricks).--'Seat thee fitly in thy firm seat!'--that is, 'Seat thee fitly in thy steady seat;'--'enjoying the first appearance of the Ukhya,'--the Ukhya, doubtless, is this Agni; and that first layer is indeed his first appearance: thus, 'enjoying that.'--'May the Asvins, the Adhvaryus, settle you here!' for the Asvins, as Adhvaryus, did lay down (this brick).