Srimad-Bhagavatam: Canto 3 - Chapter 1 - Verse 10

Sanskrit:

मनोऽचिरात्स्याद्विरजं जितश्वासस्य योगिन: ।वाय्वग्निभ्यां यथा लोहं ध्मातं त्यजति वै मलम् ॥ १० ॥

ITRANS:

mano ’cirāt syād virajaṁjita-śvāsasya yoginaḥvāyv-agnibhyāṁ yathā lohaṁdhmātaṁ tyajati vai malam

Translation:

The yogīs who practice such breathing exercises are very soon freed from all mental disturbances, just as gold, when put into fire and fanned with air, becomes free from all impurities.

Purport:

This process of purifying the mind is also recommended by Lord Caitanya; He says that one should chant Hare Kṛṣṇa. He says further, paraṁ vijayate: “All glories to Śrī Kṛṣṇa saṅkīrtana!” All glories are given to the chanting of the holy names of Kṛṣṇa because as soon as one begins this process of chanting, the mind becomes purified. Ceto-darpaṇa-mārjanam: by chanting the holy name of Kṛṣṇa one is cleansed of the dirt that accumulates in the mind. One can purify the mind either by the breathing process or by the chanting process, just as one can purify gold by putting it in a fire and fanning it with a bellows.