Mandukya Upanishad

The text maps Om's three sounds, A, U, M, onto the waking, dreaming, and deep-sleep states of consciousness, then adds a fourth, unspoken state, turiya, corresponding to the syllable's silence after it is spoken, pure awareness underlying the other three.

Despite its brevity, the Mandukya was given its own extended commentary, the Karika, by Gaudapada, a teacher in the lineage that produced Adi Shankaracharya, and its four-state framework remains a standard reference point across Advaita Vedanta for describing consciousness beyond ordinary waking experience.