SB 11.15.10

bhūta-sūkṣmātmani mayi
 tan-mātraṁ dhārayen manaḥ
aṇimānam avāpnoti
 tan-mātropāsako mama
Synonyms: 
bhūta-sūkṣma — of the subtle elements; ātmani — in the soul; mayi — in Me; tat-mātram — on the subtle, elemental forms of perception; dhārayet — one should concentrate; manaḥ — the mind; aṇimānam — the mystic perfection called aṇimā; avāpnoti — obtains; tat-mātra — in the subtle elements; upāsakaḥ — the worshiper; mama — My.
Translation: 
One who worships Me in My atomic form pervading all subtle elements, fixing his mind on that alone, obtains the mystic perfection called aṇimā.
Purport: 

Aṇimā refers to the mystic ability to make oneself smaller than the smallest and thus able to enter within anything. The Supreme Personality of Godhead is within the atoms and atomic particles, and one who perfectly fixes his mind in that subtle atomic form of the Lord acquires the mystic potency called aṇimā, by which one can enter within even the most dense matter such as stone.