Srimad Bhagavatam Canto 01, Chapter 04, Text 20

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.
Srimad Bhagavatam Canto 01, Chapter 04, Text 19
Srimad Bhagavatam Canto 01, Chapter 04, Text 21