Srimad Bhagavatam Canto 11, Chapter 21, Text 15

SB 11.21.15

mantrasya ca parijñanam
 karma-suddhir mad-arpanam
dharmah sampadyate sadbhir
 adharmas tu viparyayah
A mantra is purified when chanted with proper knowledge, and one’s work is purified when offered to Me. Thus by purification of the place, time, substance, doer, mantras and work, one becomes religious, and by negligence of these six items one is considered irreligious.
One receives a mantra from the mouth of a bona fide spiritual master, who instructs the disciple in the method, meaning and ultimate purpose of the mantra. The bona fide spiritual master in this age gives his disciple the maha-mantra, or holy names of God, Hare Krsna, Hare Krsna, Krsna Krsna, Hare Hare/ Hare Rama, Hare Rama, Rama Rama, Hare Hare. One who chants this mantra, considering himself to be the eternal servant of the Lord, gradually learns to chant offenselessly and by such purified chanting quickly achieves the highest perfection of life. The Lord here summarizes His discussion of purity and impurity, which manifest ultimately in religious and irreligious life.
Srimad Bhagavatam Canto 11, Chapter 21, Text 14
Srimad Bhagavatam Canto 11, Chapter 21, Text 16