Brihadaranyaka Upanishad 5.14.3

5.14.3

prāṇo'pāno vyāna ityaṣṭāvakṣarāṇyaṣṭākṣaram̐ ha vā ekaṃ gāyatryai padametadu haivāsyā etatsa yāvadidaṃ prāṇi tāvaddha jayati yo'syā etadevaṃ padaṃ vedāthāsyā etadeva turīyaṃ darśataṃ padaṃ parorajā ya eṣa tapati yadvai caturthaṃ tatturīyaṃ darśataṃ padamiti dadṛśa iva hyeṣa parorajā iti sarvamu hyevaiṣa raja uparyupari tapatyevam̐ haiva śriyā yaśasā tapati yo'syā etadevaṃ padaṃ veda

'Prâṇa' 'Apâna' and 'Vyâna'footnote{This word must be split so as to make three syllables.} make eight syllables, and the third foot of the Gâyatri has eight syllables. So the above three forms of the vital force constitute the third foot of the Gâyatri. He who knows the third foot of the Gâyatri to be such wins all the living beings that are in the universe. Now its Turiya, apparently visible, supramundane foot is indeed this—the sun that shines. 'Turiya' means the fourth. 'Apparently visible foot,' because he is seen, as it were. 'Supramundane,' because he shines on the whole universe as its overlord. He who knows the fourth foot of the Gâyatri to be such shines in the same way with splendour and fame.