Srimad-Bhagavatam: Canto 11 - Chapter 1 - Verse 23

Sanskrit:

पिण्डे वाय्वग्निसंशुद्धे हृत्पद्मस्थां परां मम ।अण्वीं जीवकलां ध्यायेन्नादान्ते सिद्धभाविताम् ॥ २३ ॥

ITRANS:

piṇḍe vāyv-agni-saṁśuddhehṛt-padma-sthāṁ parāṁ mamaaṇvīṁ jīva-kalāṁ dhyāyennādānte siddha-bhāvitām

Translation:

The worshiper should meditate upon My subtle form — which is situated within the worshiper’s own body, now purified by air and fire — as the source of all living entities. This form of the Lord is experienced by self-realized sages in the last part of the vibration of the sacred syllable om.

Purport:

According to Śrīla Śrīdhara Svāmī, the praṇava, or oṁkāra, has five parts: A, U, M, the nasal bindu and the reverberation (nāda). Liberated souls meditate upon the Lord at the end of that reverberation.