Muṇḍaka Upaniṣad · 2.2.11
ब्रह्मैवेदममृतं पुरस्ताद्ब्रह्म पश्चाद्ब्रह्म दक्षिणतश्चोत्तरेण । अधश्चोर्ध्वं च प्रसृतं ब्रह्मैवेदं विश्वमिदं वरिष्ठम्
brahmaivedam amṛtaṃ purastād brahma paścād brahma dakṣiṇataś cottareṇa | adhaś cordhvaṃ ca prasṛtaṃ brahmaivedaṃ viśvam idaṃ variṣṭham
Brahman is all this, immortal — Brahman in front, Brahman behind, Brahman to the south and to the north; spread below and above, Brahman is all this universe, the most excellent.
The culminating verse: Brahman as omnipresent totality.
Brahma eva idam — Brahman is all this. There is no other, there is no separation.
Amṛtam — immortal. Eternal, not subject to birth or death.
Purastāt… paścāt… dakṣiṇataḥ… uttareṇa — in front, behind, to the south, to the north. In all horizontal directions.
Adhaḥ… ūrdhvam — below and above. Completing the three-dimensionality.
Prasṛtam — spread, diffused. Not localized but omnipresent.
Viśvam idaṃ variṣṭham — all this universe, the most excellent. The whole as expression of absolute excellence.
The non-dual vision (advaita): there is no world apart from Brahman, there is no Brahman apart from the world. Form is the form of the formless; the multiple is the dance of the one.
The yogī rests in this certainty: all is Brahman, nothing else is.