Taittirīyopaniṣat 01.14

bhūriti vā ayaṃ lokaḥ। bhuvaḥ ityantarikṣam। suvarityasau lokaḥ। maha ityādityaḥ। ādityena vāva sarve lokā mahīyante।

"Bhūḥ" is 'this' world. "Bhuvaḥ" is the 'antarikṣa', the inner witness. "Suvaḥ" is 'that' world, (established in the vital breadth). "Mahaḥ" is the 'aditya', the sun, in which he is established. By the sun, indeed, all worlds are made great.

bhūriti vā agniḥ। bhuva iti vāyuḥ। suvarityādityaḥ। maha iti candramāḥ। candramasā vāva sarvāṇi jyotīṃṣi mahīyante।

"Bhūḥ" is fire; "Bhuvaḥ" is wind; "Suvaḥ" is the sun; "Mahaḥ" is the moon. Indeed, by the moon all lights are magnified.

bhūriti vā ṛcaḥ। bhuva iti sāmāni। suvariti yajūṃṣi। maha iti brahma। brahmaṇā vāva sarve vedā mahīyante।

The Ṛg Veda hymns are called "Bhūḥ", the Sāma Veda chants are "Bhuvaḥ", the Yajur Veda formulas are "Suvaḥ", and "Mahaḥ" is Brahman. By Brahman, indeed, all the Vedas are exalted.

bhūriti vai prāṇaḥ। bhuva ityapānaḥ। suvariti vyānaḥ। maha ityannam। annena vāva sarve prāṇā mahīyante।

"Bhūḥ" is verily 'prāṇa', the vital breath that is inhaled; "Bhuvaḥ" is 'apānaḥ', the exhalation, or downward-life force; "Suvaḥ" is 'vyānaḥ' the life force that digests; "Mahaḥ" is food. Indeed, by food all the life-breaths are magnified.

tā vā etāś catasraś caturdhā। catasraś catasro vyāhṛtayaḥ। tā yo veda। sa veda brahma। sarve'smai devā balim āvahanti ॥14॥

Namely, these fourfold are the four Vyāhṛtis. Whoever knows them, he knows Brahman. All the gods bring offerings to him.

Taittirīya Bhāshya 01.14

From the fourfold nature of the Supreme Person, he is said to have sixteen forms. The names of the Lord, Hari, is not distinct from the Lord, as his name is the cause for the world and beyond. He is known as 'ayaṃ' i.e. this, 'asau' i.e. that as he is established in vital breath, 'antarikṣam' as he is the inner witness, 'āditya'as he is situated in the sun, 'Vāyu' as he causes all that is in the world to move, 'Rig' because he is of the form of delight like the moon, 'Yajur' because of his sacrificial nature, 'Sāma' because he is equanimous, 'Brahmana' as he is present throughout the entire body of the Vedas, 'prāṇa' as he draws higher, 'apāna' as expels, and as 'vyāna' he digests food. Thus, this being, established in fourfold nature and sixteen forms, is called Mahācamasa, the great sacrificial ladle.
Another explanation: 'bhūḥ' is his one head represented by one syllable, 'bhuvaḥ' are his two arms represented by two syllables, and 'suvaḥ' are his two foundations (feet) represented by two syllables.
'Bhū' is named for its drive towards manifestation, 'bhuvaḥ' for its great vigour, 'suvaḥ' for its auspicious energy form, and 'mahaḥ' for its fullness and all-pervasiveness. Lord Brahma declares them as arising from the expansion of qualities.
The Vyāhṛtis have the meaning of Om; the Rig and other Vedas, including the Itihasa-Purāṇas and the Pañcarātra have the meaning of the Vyāhṛtis.

"lokajyotiḥprāṇavedeṣvekah sa puruṣottamaḥ। pratyekaśaścāturātmyāt ṣoḍaśātmā prakīrtitaḥ॥

"He, the Supreme Person, who is one, is described as the light of the world and as the life-breath in the Vedas. Individually, from his fourfold nature, he is said to have sixteen forms.

mahīyate mahati ca svayaṃ sa bhagavān hariḥ। pūjyapūjakabhedo'tra naiva kaścidapīṣyate॥ nāmapravṛttihetutvāt nāma lokādikaṃ hareḥ। ayaṃ samīpasthatayā tvasau prāṇe sthitatvataḥ॥

The Lord Hari himself is truly honored as 'mahati' i.e. the great. The name of the Lord, Hari, is not distinct from the Lord, as his name is the cause for the world and beyond. He is known as 'ayaṃ' i.e. this, due to proximity, and is known as 'asau' i.e. that, because it is established in the vital breath.

īkṣaṇād antarikṣaṃ cāthādityastho 'diteḥ sutaḥ। agnir agnau sthitatvāc ca vāyur vayati yaj jagat॥

He is known as 'antarikṣam' as he is the inner witness, and as he is situated in the Sun and is the sun of Aditi, he is called 'āditya'. He is called 'Agni' as he is established in fire, and 'Vāyu' as he causes all that is in the world to move.

candra āhlādarūpatvāt ṛgarcyatvāj janārdanaḥ। yajuryājyasvarūpatvāt sāma cāsau samatvataḥ॥

Janārdana, being of the form of delight like the moon, is praised as the Ṛg Veda; becuase of his sacrificial nature, as the Yajur Veda; because he is equal, as the Sāma Veda.

bṛhatvāt brahma vedānāṃ samudāye'khile sthitaḥ. tatsambandhād vedarāśir brahmaśabdena kīrtitaḥ॥

Because of its vastness, Brahman is present throughout the entire body of the Vedas. Therefore, due to this connection, the collection of the Vedas is referred to by the term 'Brahman'.

prakṛṣṭanayanāt prāṇo'pāno'vāṅnayanāddhariḥ। vividhaṃ nayanādvyānaḥ so'nnaṃ sarvopajīvyataḥ॥

Lord Hari, through prāṇa draws higher, and through apāna expels; through downward guidance of vyāna digests food on which every being subsists.

evaṃ ṣoḍaśarūpo'sau caturātmā vyavasthitaḥ। mahācamasanāmāsau yasmādatichamatkṛtiḥ॥

Thus, this being, established in fourfold nature and sixteen forms, is called Mahācamasa, the great sacrificial ladle, because of its extraordinary marvel.

māhācamasyastajjñānī brahmā samparikīrtitaḥ। evaṃ ṣoḍaśarūpāṇāṃ jñānayogyaścaturmukhaḥ॥

Thus the great Chamasa, the sacrificial ladle, is identified as Brahmā. In this way, the sixteen forms, the four-faced Brahmā is fit to be known.

sa eva brahmavit tasmāt pūjyate muktigo'pi san। sarvadevairatitaraṃ yassamyak ṣoḍaśātmavit॥"

He who truly knows the sixteen-fold Ātmān is indeed the knower of Brahman; therefore, even as a liberated one, he is worshipped exceedingly by all the gods."

iti vyāhṛtitattve।

- stated thus in the ancient text called Vyāhṛtitattva, i.e. the principle of the sacred utterance.

"tasya bhūriti śiraḥ। ekaṃ śiraḥ। ekam etad akṣaram। bhuva iti bāhū। dvau bāhū। dve ete akṣare। suvar iti pratiṣṭhe। dve pratiṣṭhe। dve ete akṣare॥"

"For him, 'bhūḥ' is the head, one head represented by one syllable; 'bhuvaḥ' are the arms, two arms represented by two syllables; 'suvaḥ' are the foundations, two foundations represented by two syllables."

ityādiśruteḥ।

- there are such Vedic testimonials.

sarvavyāhṛtīnāṃ pravettā māhācamāsyaḥ caturthītvēna etāṃ pravedayata iti viśēṣaḥ। bhūrnāmā sphūrtirūpatvāt bhūrivīryatvatō bhuvaḥ। suvaḥ subalarūpatvāt mahaḥ pūrṇatvatō vibhuḥ॥ tadetaccaturākāraṃ brahmoktaṃ guṇabṛṃhaṇāt।

The Mahācamasa, the one who knows all the vyāhṛtis explains 'maha' as the fourth; this is the distinction. 'Bhū' is named for its drive towards manifestation, 'bhuvaḥ' for its great vigour, 'suvaḥ' for its auspicious energy form, and 'mahaḥ' for its fullness and all-pervasiveness. This, which is four-formed, is declared by Lord Brahma as arising from the expansion of qualities.

"sa ātmā sarvadevānāṃ caturātmā janārdanaḥ। tasya brahmādayo devā aṅgamāgantukatvataḥ॥

"Lord Janārdana, who is the essence of all gods and possesses a fourfold nature, has Brahmā and the other gods as incidental parts of himself.

vāsudevādirūpo'sau ṣoḍaśātmā catuścatuḥ। sa eva sarvavedoktaḥ sarvavidyāsu ceśvaraḥ॥

He, whose forms begin with Vāsudeva, in the sixteen-fold form of himself, grouped as four and four. He alone is described in all the Vedas and is the lord of all branches of knowledge.

praṇavārthā vyāhṛtayo vyāhṛtyarthā ṛgādayaḥ। itihāsapurāṇaṃ ca pañcarātraṃ ca sarvaśaḥ॥

The Vyāhṛtis have the meaning of Om; the Rig and other Vedas have the meaning of the Vyāhṛtis; this includes the Itihasa-Purāṇas, and the Pañcarātra.

samyag vyāharaṇāt viṣṇoḥ śrutā vyāhṛtayastviti। sarvavedokta sarvasvavyāhṛtervyāhṛtitvamu। viśeṣeṇā'harantīmaṃ viṣṇumityathavā smṛtāḥ॥"

The Vyāhṛtis are the sacred utterances heard coming from Lord Viṣṇu. All the Vedas declare the all-essence Vyāhṛti, and indeed the state of being a Vyāhṛti. Especially they convey Lord Viṣṇu and are meant to remember him."

iti vyāhṛti sāre ॥14॥

- stated thus in the ancient text of "Vyāhṛti-sāre", the essence of the utterances.