Srimad-Bhagavatam: Canto 12 - Chapter 1 - Verse 47

Sanskrit:

यथा हेम्नि स्थितो वह्निर्दुर्वर्णं हन्ति धातुजम् ।एवमात्मगतो विष्णुर्योगिनामशुभाशयम् ॥ ४७ ॥

ITRANS:

yathā hemni sthito vahnirdurvarṇaṁ hanti dhātu-jamevam ātma-gato viṣṇuryoginām aśubhāśayam

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.