Taittirīyopaniṣat 01.13

bhūrbhuvassuvariti vā etās tisro vyāhṛtayaḥ tāsā mu ha smaitāṃ caturthīm। māhācamāsyaḥ pravedayate। maha iti। tad brahma। sa ātmā। aṅgāny anyā devatāḥ ॥13॥

The words 'bhūḥ', 'bhuvaḥ', and 'suvaḥ' are the three sacred utterances; indeed, the great sage Chamasa teaches the fourth, 'maha'. That is Brahman. He is the Ātmān. The other deities are his limbs.

Taittirīya Bhāshya 01.13

Lord Hari, who is the Lord of the Vedas, is clearly manifest from the immortal sounds of the Vedas. He is called 'Brahmakosha' i.e. the repository of Brahman, as he is the abode of the Vedas; is called 'Bhaga' as he is full of the six qualities; and is called 'Vibhu' as he possesses manifold forms. The four Vyahritis 'Bhū', 'Bhuvaḥ', 'Suva' and 'Maha' correspond to the Aniruddha, Pradyumna, Sankarshana, and Vāsudeva forms of the Lord, respectively. Each of them corresponds to the head, arms, feet, and the centre (heart).

"yo viśvarūpo bhagavān chandasāmadhipo hariḥ। chandobhyo'mṛtarūpebhyaḥ suvyaktastadupāsanāt ॥ medhāvī tattvavijñānī bhūtvā kṣipraṃ vimucyate॥"

"He who is Hari, the Lord of the Vedas and of universal form, is clearly manifest from the immortal forms of the Vedas through worship. Such an intelligent one, having become the knower of the truth, is quickly liberated."

iti ca।

- stated thus as well.

"vedāvāsatvato viṣṇuḥ brahmakosha iti smṛtaḥ। bhagaḥ ṣaḍguṇapūrṇatvāt bahurūpatvato vibhuḥ॥ sahasraśā kha ity uktas tam upāsya janārdanam। mucyate nātra sandehaḥ sarvabhogaiś ca yujyate॥"

Viṣṇu is called 'Brahmakosha' i.e. the repository of Brahman, because he is the abode of the Vedas (knowledge); 'Bhaga', being full of the six qualities; and 'Vibhu', as he possesses manifold forms. It is said that whoever worships Janardana, who is described as 'Sahasraśākha' i.e. thousand-branched, is surely liberated and enjoys all pleasures; there is no doubt about this.

iti praklṛpte।

- stated thus in the ancient text called 'Praklṛpte'.

"āvahantī ciraṃ sarvabhogān yā śrīssanātanī। tāṃ mayyāvaha govinda suśikhaṇḍopaśobhitām॥"

"O Govinda, bring unto me that eternal Śrī (prosperity), adorned with beautiful crest-feathers, who brings all enjoyments for a long time."

iti ca ॥12॥

- stated thus.

bhūrādi vyāhṛtībhistu vācyaṃ mūrticatuṣṭayam। aniruddhādikaṃ vāsudevāntaṃ dehamadhyagaḥ॥

The fourfold form, starting from Bhū and the other Vyāhṛtis, should be described; these forms correspond from Aniruddha to Vāsudeva (Aniruddha, Pradyumna, Sankarshana, and Vāsudeva), and are situated in the middle of the body.

vāsudevo mahonāmā tvaniruddhaśśiro mataḥ। bhūrnāmātha bhuvonāmā bāhū pradyumna īritaḥ॥

Vāsudeva is called 'Maha', the great one , but Aniruddha is regarded as the head, denoted by the sound 'Bhūḥ'. The arms are said to be Pradyumna, denoted by the sound 'Bhuvaḥ'.

saṅkarṣaṇassuvarnāmā pādau tasya mahātmanaḥ। ananyo'pyanyaśabdena caturātmā prakīrtyate॥

Saṅkarṣaṇa is regarded as the feet, denoted by the sound 'Suva'; that essence is also called 'Maha'; as he is also donated by the word 'Anya' i.e. 'another'; and is proclaimed to have four forms.

nirviśeṣo'pi bhagavān saṅkhyāmātraviśeṣataḥ ॥13॥

Even though the Lord is without distinction, He is considered distinct only by the difference in number.