Mandukya Upanishad, first verse:

Text

हरिः ओम् । ओमित्येतदक्षरमिदं सर्वं तस्योपव्याख्यानं भूतं भवद्भविष्यदिति सर्वमोङ्कार एव | यच्चान्यत्त्रिकालातीतं तदप्योङ्कार एव ॥ १ ॥

hariḥ om | omityetadakṣaramidaṃ sarvaṃ tasyopavyākhyānaṃ bhūtaṃ bhavadbhaviṣyaditi sarvamoṅkāra eva | yaccānyattrikālātītaṃ tadapyoṅkāra eva || 1 ||

1. Hariḥ Aum. Aum, the word, is all this. A clear explanation of it (is the following). All that is past, present and future is verily Aum. That which is beyond the triple conception of time, is also truly Aum.

Shankara Bhashya (commentary)

Aum, the word, is all this. As all diversified objects that we see around us, indicated by names, are not different1 from their (corresponding) names, and further as the different names are not different from Aum, therefore all this is verily Aum. As a thing is known through its name, so the highest Brahman is known through Aum alone. Therefore the highest Brahman is verily Aum. This (treatise) is the explanation of that, tasya, that is, of Aum, the word, which is of the same nature as the higher as well as the lower Brahman. Upavyākhyānam means clear explanation, because Aum is the means to the knowledge of Brahman on account of its having the closest proximity to Brahman. The word ‘Prastutam’ meaning ‘commences’ should be supplied to complete the sentence (as otherwise, it is incomplete). That which is conditioned by the triple (conceptions of) time, such as past, present and future is also verily Aum for reasons already explained. All that is beyond the three (divisions of) time, i.e., unconditioned by time, and yet known by their effects, which is called ‘Avyākṛta’, the unmanifested, etc.,—that also2 is verily Aum.