All scholars agree that Vedas should be understood completely when you interpret Vedas with Puranas and Ithihas. Where in Vedas it is stated like that?
Both Itihasa and Puranas are considered eternal texts as they also have appeared from The Supreme person's breath:
"O Maitreya,The Rg,yajur,sama and atharva vedas as well as the itihasas and the puranas all manifest from the breathing Of the Lord" (Brihad-aranyaka Upanishad 2.4.10)
"In this way,all the vedas were manifested along with kalpas,Rahasyas ,Brahmanas,Upanishads,Itihasas,Anvakhyatas and the puranas." (Gopatha Brahmana,purva 2.10)
"Indeed, Rg,Yajur, sama and Atharva are the names Of four vedas.The itihasas and puranas are the fifth veda." (Chandogya Upanishad 7.1.4)
Verses, and songs,and magic hymns,purana,sacrificial text.all the celestial Gods whose home is heaven sprang from the residue.(Atharva Veda book 11,hymn 7 verse 24).
Chandogya Upanishad 7.1.4
Nama va rig-vedo yajur-vedah
Pancamo vedanam vedah
Indeed Rig, Yajur, Sama, and Atharva are the names of the four Vedas. The Itihasas and Puranas are the fifth Veda.
Atharva Veda 11.7.24
Rcah samani chandamsi Puranam yajusa saha Ucchistaj-jajnire sarve divi Deva divi-sritahpuranam yajusa saha
The Rig, Sama, Yajur and Atharva became manifest from the Lord, along with the Puranas and all the Devas residing in the heavens.