Mandukya Upanishad, verse 12
अमात्रश्चतुर्थोऽव्यवहार्यः प्रपञ्चोपशमः शिवोऽद्वैत एवमोङ्कार आत्मैव संविशत्यात्मनाऽऽत्मानं य एवं वेद ॥ १२ ॥
amātraścaturtho'vyavahāryaḥ prapañcopaśamaḥ śivo'dvaita evamoṅkāra ātmaiva saṃviśatyātmanā''tmānaṃ ya evaṃ veda || 12 ||
12. That which has no parts (soundless), incomprehensible (with the aid of the senses), the cessation of all phenomena, all bliss and non-dual Aum, is the fourth and verily the same as the Ātman. He who knows this merges his self in the Self.
Shankara Bhashya (commentary)
The amātroḥ (soundless1) is that which has no parts (sounds, etc., or letters). This partless Aum which is the fourth, is nothing but Pure Ātman. It is incomprehensible, because both speech and mind which correspond to the name2 and the object disappear or cease; the name and the object (that is indicated by the name) which are only forms of speech and mind cease or disappear (in the partless Aum), It is the cessation3 of the (illusion of) phenomena and all4 bliss and is identical with non-duality.5 Aum, as6 thus understood, has three sounds which are the same as the three quarters and therefore Aum is identical7 with Ātman. He who knows this merges8 his self in the Self which is the Highest Reality. Those who know Brahman, i.e., those who realise the Highest Reality merge into Self, because in their case the notion of the cause which corresponds to the third quarter (of Ātman) is destroyed (burnt). They9 are not born again, because Turīya is not a cause. For, the illusory snake which has merged in the rope on the discrimination of the snake from the rope, does not reappear as before, to those who know the distinction between them, by any effort10 of the mind (due to the previous impressions). To the men of dull or mediocre intellect who still consider themselves as students of philosophy, who having renounced the world, tread on the path of virtue and who know the common features between the sounds (mātrāḥ) and the quarters (or parts) as described above,—to them Aum, if meditated upon in a proper way, becomes a great11 help to the realisation of Brahman. The same is indicated in the Kārikā later on thus: “The three inferior stages of life, etc” (Māṇḍūkya Kārikā, Advaita Chapter, 16.)