SB 3.12.39
itihasa-puranani
pañcamam vedam isvarah
sarvebhya eva vaktrebhyah
sasrje sarva-darsanah
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Then he created the fifth Veda — the Puranas and the histories — from all his mouths, since he could see all the past, present and future.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
There are histories of particular countries and nations and of the world, but the Puranas are the histories of the universe, not only in one millennium but in many kalpas. Brahma has knowledge of those historical facts, and therefore all the Puranas are histories. As originally composed by Brahma, they are part of the Vedas and are called the fifth Veda.