The four divisions of the original sources of knowledge [the Vedas] were made separately. But the historical facts and authentic stories mentioned in the Purāṇas are called the fifth Veda.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
ऋग्यजुःसामाथर्वाख्या वेदाश्चत्वार उद्धृताः । इतिहासपुराणं च पञ्चमो वेद उच्यते
ṛgyajuḥsāmātharvākhyā vedāścatvāra uddhṛtāḥ | itihāsapurāṇaṃ ca pañcamo veda ucyate
SB 1.4.20