MĀṆḌŪKYA UPANISHAD · 12

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

amātraścaturtho’vyavahāryaḥ prapañcopaśamaḥ śivo’dvaita
evamoṅkāra ātmaiva saṃviśatyātmanā’’tmānaṃ ya evaṃ veda ya evaṃ veda ॥ 12॥

Letterless is the fourth, the Incommunicable, the end of phenomena, the Good, the One than Whom there is no other; thus is OM. He that knoweth is the Self and entereth by his self into the Self, he that knoweth, he that knoweth.

Sri Aurobindo · CWSA 18, p. 196