CHĀNDOGYA UPANISHAD · 1.3.2

समान उ एवायं चासौ च ।
उष्णो ऽयम् उष्णो ऽसौ ।
स्वर इतीमम् आचक्षते स्वर इति प्रत्यास्वर इत्य् अमुम् ।
तस्माद् वा एतम् इमम् अमुं च उद्गीथम् उपासीत ॥

samāna u evāyaṃ cāsau ca ।
uṣṇo ’yam uṣṇo ’sau ।
svara itīmam ācakṣate svara iti pratyāsvara ity amum ।
tasmād vā etam imam amuṃ ca udgītham upāsīta ॥

This (the breath in the mouth) and that (the sun) are the same. This is hot and that is hot. This they call svara (sound), and that they call pratyâsvara (reflected sound). Therefore let a man meditate on the udgîtha (Om) as this and that (as breath and as sun).

F. Max Müller · The Upanishads, Sacred Books of the East 1 (1879)