SB 12.6.37
suta uvaca
samahitatmano brahman
brahmanah paramesthinah
hrdy akasad abhun nado
vrtti-rodhad vibhavyate
Translation:
Suta Gosvami said: O brahmana, first the subtle vibration of transcendental sound appeared from the sky of the heart of the most elevated Lord Brahma, whose mind was perfectly fixed in spiritual realization. One can perceive this subtle vibration when one stops all external hearing.
Purport:
Because Srimad-Bhagavatam is the supreme Vedic literature, the sages headed by Saunaka desired to trace out its source.