CHĀNDOGYA UPANISHAD · 1.1.7
आपयिता ह वै कामानां भवति य एतद् एवं विद्वान् अक्षरम् उद्गीथम् उपास्ते ॥
āpayitā ha vai kāmānāṃ bhavati ya etad evaṃ vidvān akṣaram udgītham upāste ॥
Thus he who knowing this, meditates on the syllable (Om), the udgîtha, becomes indeed a fulfiller of desires.
F. Max Müller · The Upanishads, Sacred Books of the East 1 (1879)