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.