Mandukya 1
ओमित्येतदक्षरमिदं सर्वं तस्योपव्याख्यानम्
Om ityetad-akṣaram idaṁ sarvaṁ tasyopavyākhyānam
Aum — this syllable is all this. Its explanation: what was, what is, what will be — all is Aum.
Atharva Veda · Atharva Veda · 12 mantras
Mandukya Upanishad
Māṇḍūkyopaniṣad
Central theme: Om and the four states of consciousness
Mahavakya
अयमात्मा ब्रह्म
Ayam ātmā brahma
“This Self is Brahman.”
— Mandukya 2
In just 12 mantras, the entire teaching of Vedanta. All this is Aum. Aum has four parts: A (waking, vaishvanara), U (dream, taijasa), M (deep sleep, prajna), and the silent fourth (turiya) — pure consciousness, the Self. Gaudapada's Karika expanded this into the foundational ajatavada (non-origination) doctrine that became the seed of Advaita Vedanta. The Muktika Upanishad declares that the Mandukya alone is enough for liberation.
Mandukya 1
ओमित्येतदक्षरमिदं सर्वं तस्योपव्याख्यानम्
Om ityetad-akṣaram idaṁ sarvaṁ tasyopavyākhyānam
Aum — this syllable is all this. Its explanation: what was, what is, what will be — all is Aum.
12 mantras, all Vedanta. Sit with the silent fourth.