SB 12.3.47
yatha hemni sthito vahnir
durvarnam hanti dhatu-jam
evam atma-gato visnur
yoginam asubhasayam
Translation:
Just as fire applied to gold removes any discoloration caused by traces of other metals, Lord Visnu within the heart purifies the minds of the yogis.
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.