Srimad Bhagavatam Canto 12, Chapter 06, Text 37

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.
Srimad Bhagavatam Canto 12, Chapter 06, Text 36
Srimad Bhagavatam Canto 12, Chapter 06, Text 38