In the Taittiriya Upanisad (3.1): yato va imani bhutani jayante, yena jatani jivanti, yat prayanty abhisamvisanti, tad vijijnasasva tad brahma. This was the answer given by father Varuna when questioned by his son Varuni Bhrgu about the Absolute Truth

Śrīla Bhaktivinoda Ṭhākura states in his Amṛta-pravāha-bhāṣya that according to the injunction of the Upaniṣads (“the Supreme Absolute Truth is He from whom everything emanates”), it is understood that the whole cosmic manifestation emanated from Brahman, the Supreme Absolute Truth. The creation subsists by the energy of the Supreme Brahman and, after annihilation, merges into the Supreme Brahman. From this we can understand that the Absolute Truth can be categorized in three cases—ablative, instrumental and locative. According to these three cases, the Absolute Truth is positively personified. In this connection, Śrīla Bhaktisiddhānta Sarasvatī quotes the Aitareya Upaniṣad (1.1.1): ātmā vā idam eka evāgra āsīn nānyat kiñcana miṣat sa īkṣata lokān nu sṛjā iti. Similarly, in the Śvetāśvatara Upaniṣad (4.9) it is stated:

And in the Taittirīya Upaniṣad (3.1): yato vā imāni bhūtāni jāyante, yena jātāni jīvanti, yat prayanty abhisaṁviśanti, tad vijijñāsasva tad brahma. This was the answer given by father Varuṇa when questioned by his son Vāruṇi Bhṛgu about the Absolute Truth. In this mantra, the word yataḥ, the Absolute Truth from which the cosmic manifestation has emanated, is in the ablative case; that Brahman by which this universal creation is maintained is in the instrumental case (yena); and that Brahman into which the whole cosmic manifestation merges is in the locative case (yat or yasmin). It is stated in Śrīmad-Bhāgavatam (SB 1.5.20):

“The entire universal creation is contained in the gigantic form of the Supreme Personality of Godhead. Everything emanates from Him, everything rests in His energy, and after annihilation everything merges into His person.”