SB 1.4.20
rg-yajuh-samatharvakhya
vedas catvara uddhrtah
itihasa-puranam ca
pañcamo veda ucyate
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
The four divisions of the original sources of knowledge [the Vedas] were made separately. But the historical facts and authentic stories mentioned in the Puranas are called the fifth Veda.