Mandukya Upanishad

OṂ—this whole world is that syllable!

Accordingly, the very self is OṂ.

The Mandukya Upanishad is one of the principal Upanishads. It is a short upanishad about Om.

Quotes

Olivelle, Patrick (1998). The Early Upanishads. Oxford: Oxford University Press. ISBN 0-19-512435-9. 

• OṂ—this whole world is that syllable!

Here is a further explanation of it.

The past, the present, and the future—all that is simply OṂ;

and whatever else that is beyond the three times, that also is simply OṂ—

for this brahman is the Whole.

Brahman is this self (ātman) ("ayam ātmā brahma").

• Verses 1–2

• Accordingly, the very self (ātman) is OṂ.

Anyone who knows this enters the self (ātman) by himself (ātman).

• Verse 12