Śatapatha Brāhmaṇa ("of a hundred paths") 1.3.2.16

1.3.2.16

tāni vā etāni | candobhya ājyāni gṛhyante sa yaccaturjuhvāṃ gṛhṇāti gāyatryai tadgṛhṇātyatha yadaṣṭau kṛtva upabhṛti gṛhṇāti triṣṭubjagatībhyāṃ tadgṛhṇātyatha yaccaturdhruvāyāṃ gṛhṇātyanuṣṭubhe tadgṛhṇāti vāgvā anuṣṭubvāco vā idaṃ sarvam prabhavati tasmādu dhruvāya eva sarvā yajñaḥ prabhavatīyam vā anuṣṭubasyai vā idaṃ sarvam prabhavati tasmādu dhruvāyā eva sarvo yajñaḥ prabhavati

These butter-portions, then, are taken for the metres. Now what he takes in the guhû (by ladling) four times (with the sruva), that he takes for the gâyatrî; and what he takes in the upabhrit (by ladling) eight times, that he takes for the trishtubh and gagatî; and what he takes in the dhruvâ (by ladling) four times, that he takes for the anushtubh. For the anushtubh is speech, and from speech all this (universe) springs: hence it is from the dhruvâ that the whole sacrifice originates. The anushtubh also is this (earth), and from it all this (universe) originates: hence it is from the dhruvâ that the whole sacrifice originates.