He began to reply to the inquiries of Mahārāja Parīkṣit by saying that the science of the Personality of Godhead was spoken first by the Lord Himself to Brahmā when he was first born. Śrīmad-Bhāgavatam is the supplementary Vedic literature, and it is just in pursuance of the Vedas.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
प्राह भागवतं नाम पुराणं ब्रह्मसम्मितम् । ब्रह्मणे भगवत्प्रोक्तं ब्रह्मकल्प उपागते
prāha bhāgavataṃ nāma purāṇaṃ brahmasammitam | brahmaṇe bhagavatproktaṃ brahmakalpa upāgate
SB 2.8.28