BṚHADĀRAṆYAKA UPANISHAD · 5.14.3

प्राणो ऽपानो व्यान इत्य् अष्टाव् अक्षराणि ।
अष्टाक्षरं ह वा एकं गायत्र्यै पदम् ।
एतद् उ हैवास्या एतत् ।
स यावद् इदं प्राणि तावद् ध जयति यो ऽस्या एतद् एवं पदं वेद ।
अथास्या एतद् एव तुरीयं दर्शतं पदं परोरजा य एष तपति ।
यद् वै चतुर्थं तत् तुरीयम् ।
दर्शतं पदम् इति ददृश इव ह्य् एषः ।
परोरजा इति सर्वम् उ ह्य् एवैष रज उपर्य् उपरि तपति ।
एवं हैव श्रिया यशसा तपति यो ऽस्या एतद् एवं पदं वेद ॥

prāṇo ’pāno vyāna ity aṣṭāv akṣarāṇi ।
aṣṭākṣaraṃ ha vā ekaṃ gāyatryai padam ।
etad u haivāsyā etat ।
sa yāvad idaṃ prāṇi tāvad dha jayati yo ’syā etad evaṃ padaṃ veda ।
athāsyā etad eva turīyaṃ darśataṃ padaṃ parorajā ya eṣa tapati ।
yad vai caturthaṃ tat turīyam ।
darśataṃ padam iti dadṛśa iva hy eṣaḥ ।
parorajā iti sarvam u hy evaiṣa raja upary upari tapati ।
evaṃ haiva śriyā yaśasā tapati yo ’syā etad evaṃ padaṃ veda ॥

The Prâna (the up-breathing), the Apâna (the down-breathing), and the Vyâna (the back-breathing) form eight syllables. One foot (the third) of the Gâyatrî consists of eight syllables. This (one foot) of it is that (i. e. the three vital breaths). And he who thus knows that foot of it, conquers as far as there is anything that breathes. And of that (Gâyatrî, or speech) this indeed is the fourth (turîya), the bright (darsata) foot, shining high above the skies. What is here called turîya (the fourth) is meant for katurtha (the fourth); what is called darsatam padam (the bright foot) is meant for him who is as it were seen (the person in the sun); and what is called paroragas (he who shines high above the skies) is meant for him who shines higher and higher above every sky. And he who thus knows that foot of the Gâyatrî, shines thus himself also with happiness and glory.

F. Max Müller · The Upanishads, Sacred Books of the East 15 (1884)