SB 11.27.23

piṇḍe vāyv-agni-saṁśuddhe
 hṛt-padma-sthāṁ parāṁ mama
aṇvīṁ jīva-kalāṁ dhyāyen
 nādānte siddha-bhāvitām
Word for word: 
piṇḍe — within the body; vāyu — by air; agni — and by fire; saṁśuddhe — which has become completely purified; hṛt — of the heart; padma — upon the lotus; sthām — situated; parām — the transcendental form; mama — of Mine; aṇvīm — very subtle; jīva-kalām — the Personality of Godhead, from whom all living entities expand; dhyāyet — he should meditate upon; nāda-ante — at the end of the vibration of om; siddha — by perfected sages; bhāvitām — experienced.
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.