SB 12.3.47

yathā hemni sthito vahnir
 durvarṇaṁ hanti dhātu-jam
evam ātma-gato viṣṇur
 yoginām aśubhāśayam
Synonyms: 
yathā — just as; hemni — in gold; sthitaḥ — situated; vahniḥ — fire; durvarṇam — the discoloration; hanti — destroys; dhātu-jam — due to the taint of other metals; evam — in the same way; ātma-gataḥ — having entered the soul; viṣṇuḥ — Lord Viṣṇu; yoginām — of the yogīs; aśubha-āśayam — the dirty mind.
Translation: 
Just as fire applied to gold removes any discoloration caused by traces of other metals, Lord Viṣṇu within the heart purifies the minds of the yogīs.
Purport: 

Although one may practice the mystic yoga system, his actual spiritual advancement is due to the mercy of the Supreme Lord within the heart; it is not directly the result of his austerity and meditation. If one becomes foolishly proud in the name of yoga, his spiritual position becomes ridiculous.