Śatapatha Brāhmaṇa ("of a hundred paths") 10.3.1.1

10.3.1.1

prāṇo gāyatrī cakṣuruṣṇigvāganuṣṭummano bṛhatī śrotram paṅktirya evāyam prajananaḥ prāṇa eṣa triṣṭubatha yo'yamavāṅ prāṇa eṣa jagatī tāni vā etāni sapta candāṃsi caturuttarāṇyagnau kriyante

The Gâyatrî is the breath (of Pragâpati, the altar), the Ushnih the eye, the Anushtubh the voice, the Brihatî the mind, the Paṅkti the ear; the Trishtubh is that generative breath; and the Gagatî that downward breathing;--these are the seven metres increasing by four (syllables) each, which are produced in Agni (the fire-altar).