SB 12.6.39

tato ’bhūt tri-vṛd oṁkāro
 yo ’vyakta-prabhavaḥ sva-rāṭ
yat tal liṅgaṁ bhagavato
 brahmaṇaḥ paramātmanaḥ
tataḥ — from that; abhūt — came into being; tri-vṛt — threefold; oṁkāraḥ — the syllable om; yaḥ — which; avyakta — not apparent; prabhavaḥ — its influence; sva-rāṭ — self-manifesting; yat — which; tat — that; liṅgam — the representation; bhagavataḥ — of the Supreme Personality of Godhead; brahmaṇaḥ — of the Absolute Truth in His impersonal aspect; parama-ātmanaḥ — and of the Supersoul.
From that transcendental subtle vibration arose the oṁkāra composed of three sounds. The oṁkāra 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.