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.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
ततोऽभूत्त्रिवृदोङ्कारो योऽव्यक्तप्रभवः स्वराट् । यत्तल्लिङ्गं भगवतो ब्रह्मणः परमात्मनः
tato'bhūttrivṛdoṅkāro yo'vyaktaprabhavaḥ svarāṭ | yattalliṅgaṃ bhagavato brahmaṇaḥ paramātmanaḥ
SB 12.6.39