Narada Parivrajaka Upanishad of Atharvaveda, Chapter VIII

Then Nārada, asked Parameshthī (Brahmā) to enlighten him, who had surrendered himself to Him, about saṃsāra-tāraka (or that tāraka or Praṇava which lifts one out of saṃsāra).

Assenting to which, Brahmā began thus: "Omkāra that is Brahman is the vyashti (individual) and the samashti (cosmic). What is the individual? What is the cosmic? Brahma-praṇava is of three kinds, samhāra- (destructive) praṇava, sṛṣṭi- (creative) praṇava, and ubhayātmaka (belonging to both) praṇava, as being of two forms, internal and external. (It is also eight:) Antaḥ-praṇava, Vyāvahārika-praṇava, bāhya-praṇava, ārsha-praṇava, ubhayātmaka or virāt-praṇava, samhāra-praṇava, brahma-praṇava, and ardhamātrā praṇava. Om is Brahman. Know that the mantra of the one-syllabled Om is Praṇava. It has the eight differences of akāra, ukāra, makāra, ardhamātrā, nāda, bindu, kalā, and śakti. Know it is not four (alone). Akāra is associated with ten thousand limbs; ukāra, with one thousand limbs; makāra with one hundred limbs; ardhamātrā is of the nature of endless limbs. That which is saguṇa (associated with guṇas) is virāt- (preservation) praṇava; that which is nirguṇa (not associated with guṇas) is samhāra- (or destruction) praṇava; that which is associated with guṇas and is not so associated, is utpatti- (or origination) praṇava. Pluta (the elongated accent) is virāt: plutapluta is samhāra. The virāt-praṇava is of the form of sixteen mātrās and is above the thirty-six tattvas. The sixteen mātrās are thus: Akāra is the first mātrā; ukāra is the second; makāra is the third; ardhamātrā is the fourth; nāda is the fifth; bindu is the sixth; kalā is the seventh; kalātīta is the eighth; śānti is the ninth; śāntyatita is the tenth; unmanī is the eleventh; manonmanī is the twelfth; purītati is the thirteenth; tanumadhyamā is the fourteenth; pati is the fifteenth; parā is the sixteenth. Then (again) having sixty-four mātrās and their division into the two, Prakṛti and Puruṣa and resolving themselves into the one hundred and twenty-eight differences of mātrās, it becomes saguṇa and nirguṇa. Though Brahma-praṇava is one only, it is the substratum of all, the support of the whole universe, of the form of all akṣaras (letters), time, Vedas, and Śiva. This Omkāra should be sought after, that is mentioned in the Vedas of the nature of the Upaniṣads. Know that this Omkāra is the Ātmā that is indestructible during the three periods of time, past, present, and future, able to confer salvation and eulogized by Brahma-sound (Vedas). Having experienced this one Om as immortal and ageless, and having brought about the Brahma-nature in this body, become convinced that your Ātmā, associated with the three bodies, is Parabrahman. Through Viśva and others (viz., Taijasa, Prājña, and Turya) in order, the realisation of Parabrahman should be attained, since Ātmā is of four kinds through his identification with, and the enjoying of, the gross as well as the enjoyer of the gross, the subtle as well as the enjoyer of the subtle, and through his identification (with the third body) enjoying bliss in the fourth. He has four feet. The one presiding over the waking state is gross; and since he is the enjoyer of Viśva (the universe), he becomes the sthūla-prajñā (gross consciousness). He has nineteen^[1] facets and eight parts. He is pervading everywhere and the Lord. He is the enjoyer of the gross and is the caturātma called Viśva. He alone is the Puruṣa called Vaiśvānara. He alone is Viśvajit (the conqueror of the universe). This is the first foot. When this Lord attains the dreaming condition, he is the sūkṣma-prajña (subtle consciousness). O conqueror of all, he is the one having eight limbs, and there is none else. He is the enjoyer of the subtle and is caturātma, named Taijasa and the protector of elements. He alone is the Hiraṇyagarbha, presiding over the gross (or subtle matter rather). He is said to form the second foot. Suṣupti (or the dreamless sleep) is that state where one sleeps without any desire and where one sees not any dreams. The one identified with this dreamless sleep is Prajñāna-ghana, is blissful, of the nature of eternal bliss and the Ātmā in all creatures; yet he is enjoyer of bliss, has cetas (consciousness) as his (one) foot, is all-pervading, indestructible, caturātmā and the Lord, and is named Prājña, the third foot. He alone is the Lord of all, the knower of all, the subtle-thoughted, the latent one, and the cause of all creation. He alone is the origin and the destruction. These three (states) are obstacles to all creatures obtaining (the final) peace. As is svapna, so is suṣupti, it (also) being said to be illusory. The caturātmā, the fourth, as he is Sat, Chit and Ēkarasa (the one essence), ends as the fourth and follows (upon the heels of each of the above states), is the knower of the means of vikalpa-jñāna and is the anujñātā (the one following knower). Having known them, and known as māyā the three vikalpas of suṣupti, svapna and āntara (the inner), even in this state, is he not (to be known as) Sat-Chit-Ēkarasa? This shall be expressed as differentiated thus: It is not even the gross prajñā; nor is it the very subtle prajñā; nor is it prajñā itself (of the causal body): O muni neither is it the trifling prajñā; nor is it the non-prajñā; nor is it the dual prajñā; nor is it the internal prajñā, though it is without prajñā; it is Prajñāna-ghana. It can never be known by the organs; nor it can be known by the reason; it cannot be grasped by the organs of action. It cannot be proved. It cannot be reached by thought. It cannot be proved by analogy. It can be realised by Self-realisation alone. It is with the waking state, etc. It is the auspicious, with changes, without a second. Such a one is thought to be Turya. This alone is Brahman, Brahma-praṇava. This should be known. There is no other Turya. To the aspirants after salvation, it is the support, like the sun everywhere; it is the Self-light. As it alone is Brahman, this Brahma-Akās is shining always. Thus is the Upaniṣad."