Srimad-Bhagavatam: Canto 11 - Chapter 1 - Verse 34

Sanskrit:

हृद्यविच्छिन्नमोङ्कारं घण्टानादं बिसोर्णवत् ।प्राणेनोदीर्य तत्राथ पुन: संवेशयेत् स्वरम् ॥ ३४ ॥

ITRANS:

hṛdy avicchinam oṁkāraṁghaṇṭā-nādaṁ bisorṇa-vatprāṇenodīrya tatrāthapunaḥ saṁveśayet svaram

Translation:

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.

Purport:

It appears that the yoga system is somewhat technical and difficult to perform. Anusvāra refers to a nasal vibration pronounced after the fifteen Sanskrit vowels. The complete explanation of this process is extremely complicated and obviously unsuitable for this age. From this description we can appreciate the sophisticated achievements of those who in former ages practiced mystic meditation. Despite such appreciation, however, we should stick firmly to the simple, foolproof method of meditation prescribed for the present age, the chanting of Hare Kṛṣṇa, Hare Kṛṣṇa, Kṛṣṇa Kṛṣṇa, Hare Hare/ Hare Rāma, Hare Rāma, Rāma Rāma, Hare Hare.