Śrīmad-Bhāgavatam · Text 11.14.34
Who is speaking and story frame
हृद्यविच्छिन्नमोङ्कारं घण्टानादं बिसोर्णवत् ।
प्राणेनोदीर्य तत्राथ पुन: संवेशयेत् स्वरम् ॥ ३४ ॥
hṛdy avicchinam oṁkāraṁ
ghaṇṭā-nādaṁ bisorṇa-vat
prāṇenodīrya tatrātha
punaḥ saṁveśayet svaram
Beginning from the mūlādhāra-cakra, one should move the life air continuously upward like the fibers in the lotus stalk until one reaches the heart, where the sacred syllable om is situated like the sound of a bell. One should thus continue raising the sacred syllable upward the distance of twelve aṅgulas, and there the oṁkāra should be joined together with the fifteen vibrations produced with anusvāra.
Read the purport on Vedabase.ioFrom Canto 10, chapter 14 onward, the translation and purports were completed by Śrīla Prabhupāda's disciples after his departure, and published by the Bhaktivedanta Book Trust.