Anandakanda 1.2.102

1.2.102

yaṃbījenātmano dehaṃ mūlādhārādviśoṣayet /
ṣaḍādhārāṇi nirbhidya yāvat brahmabilaṃ priye //

With the seed-syllable 'yaṃ', one should desiccate one's own body from the mūlādhāra,
piercing the six bodily centers as far as the cavity of Brahman, O beloved.