Chandogya Upanishad says:
Sarvam Khalu Idam Brahma – everything is verily Brahman.
Brahadaranyaka Upanishad says:
aham brahma asmi – I am Brahman.
Ishavaasya Upanishad says:
ishaavasyam idam sarvam yat kincha jagatyaam jagat – whatever moves or not moves in the world, all these are pervaded by Ishwara or Brahman alone.
Mundaka Upanishad says:
brahmaivedham amritam purastaat brahma
paschaat brahma dakshinahtascha uttarena
adhascha urdhvam cha prasritham
brahmaiva idam vishvam idam varishtham
Brahman is of the nature of immortality and it is present here everywhere. Brahman is in the front, Brahman is in the back, Brahman is in left, Brahman is in right, Brahman is on top and Brahman is on bottom – everything in the world is filled with Brahman which is great and attainable or desirable object.
Taittiriya Upanishad says:
Yena sarvaani bhootaani jaayanthe
Yena jaathaani jeevanthi
Tad vijijnaasasva tad brahma ithi
That from which all objects have come, that in which all objects exist and that into which all objects merge during destruction – that is to be known, know that to be Brahman.
The above shloka indirectly says that the world is Brahman alone as the world which is arising and merging into the substratum of Brahman is only temporary and such objects which rise and merge is called illusion in the changeless substratum of Brahman where no such change is possible.
The import of the above sloka is mentioned in the very second sutra of Brahma Sutra as
Janmaadi asya yathah – That from which the world has come (sristhi, sthithi and laya all are meant here by the word Aadi or etc.) – that is Brahman.
Sri Krishna says in Gita Chapter 7:
Mattah parataram na anyath kinchit asthi dhananjaya – O Arjuna, there is nothing different from me here.
Vishnu Purana states
Asya Jagat cha sah – this world also is Brahman alone.
Srimad Bhagavatham states in Chatusutri Bhagavatham
Aham eva aasam eva agre na anyat sad asad param
Paschaath aham yat etat chet yo avashishyet sosmi aham
I alone was present previously – there was nothing either existent or non-existent different from me. Afterwards (now) I alone am being seen as the changing and illusory world. After this illusion of world ends, I alone will remain behind (Here “I” means the ultimate reality of Brahman or Consciousness as this statement is made by the Lord Vishnu who is the embodiment of Brahman in Srimad Bhagavatham).
(Source: Excerpts from an article on Advaita Pancharatnam)