The Mukhya Upaniṣads

Māṇḍūkya · Om and the Four Quarters of the Self

Mantra 9

जागरितस्थानो वैश्वानरोऽकारः प्रथमा मात्राऽऽप्तेरादिमत्त्वाद् वाऽऽप्नोति ह वै सर्वान् कामानादिश्च भवति य एवं वेद ॥ ९ ॥

In plain words

Vaiśvānara, whose field is waking, is the first sound-unit A, because of pervasion or primacy. One who knows thus attains worthy desires and becomes foremost.