SB 6.2.18

ajñānād athavā jñānād
 uttamaśloka-nāma yat
saṅkīrtitam aghaṁ puṁso
 dahed edho yathānalaḥ
Synonyms: 
ajñānāt — out of ignorance; athavā — or; jñānāt — with knowledge; uttamaśloka — of the Supreme Personality of Godhead; nāma — the holy name; yat — that which; saṅkīrtitam — chanted; agham — sin; puṁsaḥ — of a person; dahet — burns to ashes; edhaḥ — dry grass; yathā — just as; analaḥ — fire.
Translation: 
As a fire burns dry grass to ashes, so the holy name of the Lord, whether chanted knowingly or unknowingly, burns to ashes, without fail, all the reactions of one’s sinful activities.
Purport: 

Fire will act, regardless of whether handled by an innocent child or by someone well aware of its power. For example, if a field of straw or dry grass is set afire, either by an elderly man who knows the power of fire or by a child who does not, the grass will be burned to ashes. Similarly, one may or may not know the power of chanting the Hare Kṛṣṇa mantra, but if one chants the holy name he will become free from all sinful reactions.