Chhāndogya Upanishad verses 7.1.2 and 7.1.4 mention the Itihasa and Puranas as the 5th Veda.
It also mentions about Lord Krishna the son of devakI.
Another verse from brihadaranyaka mentions vedas and upanishads itihasa etc separately. Which shows vedas were already arranged into upanishad brahmanas etc.
Here is the BrihadaranyAkopanishad's verse that mentions the four Vedas and Upanishads separately:
The King Janaka asks YajnavAlkya "KA prajnata yAjnavalkya|" OR "What is PrajnA, YajnavAlkya?"
To that YajnavAlkya replies:
VAgeva samrariti hovAchA | VAchA vai samrAr vandhuh prajnAyatarigvedo yajurvedha sAmavedah atharvangirasa itihAsah purAnam vidyA upanishadshlokAh sutrAnya anuvyAkhyAnAni ... vAgvai samrAt param brahmam ||
The meaning is:
O king, the vAk is the prajnA. By VAk a firend can be known. By VAk the Rig Veda, the Yajur Veda, the SAma Veda and the Atharva Veda, the ItihAsas, the PurAnas, the VidyAs, the Upanishads, the Shlokas, Sutras, the explanations of them and the supplementary explanations (anu vyAkhyAni) .. etc can be understood. That VAk is the Param Brahman.
From the Upanishad's 4th adhyAya's 1st BrAhmana (called the ShadAchArya).
Since I discussed in this question about classification of the Vedas into four, people came up with references which show that the Vedas were classified into FOUR somewhere towards the end of dwapara yuga.
People came up with reasoning that the Purana and itihasa mentioned in the chandogya are actually not the texts like mahabharata, ramayana, and puranas written by Vyasa but it talks of events that happened in the past. Even if that is accepted, we still get a major question here.
Were the upanishads written when the vedas were already classified into FOUR during the end of dwapara yuga?
If not then how come it talks of the fifth veda?
Can it be concluded that the major upanishads (as chandogya is one of the oldest upanishads) were written around the era of veda vyasa ?
PS: Some people believe that everything repeats in every kalpa. However, they come into existence only when the event actually occurs in that particular yuga. Else the text should mention if it talks about prophesies of upcoming yugas.