The Mukhya Upaniṣads

Māṇḍūkya · Om and the Four Quarters of the Self

Mantra 12

अमात्रश्चतुर्थोऽव्यवहार्यः प्रपञ्चोपशमः शिवोऽद्वैत एवमोङ्कार आत्मैव संविशत्यात्मनाऽऽत्मानं य एवं वेद य एवं वेद ॥ १२ ॥

In plain words

The soundless fourth is beyond transaction, the cessation of phenomenal projection, auspicious, non-dual. Thus Om is the Self itself. One who knows thus enters the Self by the Self.