CHĀNDOGYA UPANISHAD · 1.2.14

आगाता ह वै कामानां भवति य एतद् एवं विद्वान् अक्षरम् उद्गीथम् उपास्ते ।
इत्य् अध्यात्मम् ॥

āgātā ha vai kāmānāṃ bhavati ya etad evaṃ vidvān akṣaram udgītham upāste ।
ity adhyātmam ॥

He who knows this, and meditates on the syllable Om (the imperishable udgîtha) as the breath of life in the mouth, he obtains all wishes by singing. So much for the udgîtha (Om) as meditated on with reference to the body.