Srimad-Bhagavatam: Canto 12 - Chapter 1 - Verse 39
Sanskrit:
ततोऽभूत्त्रिवृदोंकारो योऽव्यक्तप्रभव: स्वराट् ।यत्तल्लिङ्गं भगवतो ब्रह्मण: परमात्मन: ॥ ३९ ॥
ITRANS:
tato ’bhūt tri-vṛd oṁkāroyo ’vyakta-prabhavaḥ sva-rāṭyat tal liṅgaṁ bhagavatobrahmaṇaḥ paramātmanaḥ
Translation:
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.