CHĀNDOGYA UPANISHAD · 1.2.1

देवासुरा ह वै यत्र संयेतिरे ।
उभये प्राजापत्यास् तद् ध देवा उद्गीथम् आजह्रुर् अनेनैनान् अभिभविष्याम इति ॥

devāsurā ha vai yatra saṃyetire ।
ubhaye prājāpatyās tad dha devā udgītham ājahrur anenainān abhibhaviṣyāma iti ॥

When the Devas and Asuras struggled together, both of the race of Pragâpati, the Devas took the udgîtha (Om), thinking they would vanquish the Asuras with it.

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