Srimad Bhagavatam Canto 12, Chapter 06, Text 39

SB 12.6.39

tato ’bhut tri-vrd omkaro
 yo ’vyakta-prabhavah sva-rat
yat tal lingam bhagavato
 brahmanah paramatmanah
 
Translation: 
 
From that transcendental subtle vibration arose the omkara composed of three sounds. The omkara has unseen potencies and manifests automatically within a purified heart. It is the representation of the Absolute Truth in all three of His phases — the Supreme Personality, the Supreme Soul and the supreme impersonal truth.
Srimad Bhagavatam Canto 12, Chapter 06, Text 38
Srimad Bhagavatam Canto 12, Chapter 06, Text 40-41