SB 6.5.27-28

ab-bhakṣāḥ katicin māsān
 katicid vāyu-bhojanāḥ
ārādhayan mantram imam
 abhyasyanta iḍaspatim
oṁ namo nārāyaṇāya
 puruṣāya mahātmane
viśuddha-sattva-dhiṣṇyāya
 mahā-haṁsāya dhīmahi
Synonyms: 
ap-bhakṣāḥ — drinking only water; katicit māsān — for some months; katicit — for some; vāyu-bhojanāḥ — merely breathing, or eating air; ārādhayan — worshiped; mantram imam — this mantra, which is nondifferent from Nārāyaṇa; abhyasyantaḥ — practicing; iḍaḥ-patim — the master of all mantras, Lord Viṣṇu; om — O Lord; namaḥ — respectful obeisances; nārāyaṇāya — unto Lord Nārāyaṇa; puruṣāya — the Supreme Person; mahā-ātmane — the exalted Supersoul; viśuddha-sattva-dhiṣṇyāya — who is always situated in the transcendental abode; mahā-haṁsāya — the great swanlike Personality of Godhead; dhīmahi — we always offer.
Translation: 
For a few months the sons of Prajāpati Dakṣa drank only water and ate only air. Thus undergoing great austerities, they recited this mantra: “Let us offer our respectful obeisances unto Nārāyaṇa, the Supreme Personality of Godhead, who is always situated in His transcendental abode. Since He is the Supreme Person [paramahaṁsa], let us offer our respectful obeisances unto Him.”
Purport: 

From these verses it is apparent that the chanting of the mahā-mantra or the Vedic mantras must be accompanied by severe austerities. In Kali-yuga, people cannot undergo severe austerities like those mentioned herein — drinking only water and eating only air for many months. One cannot imitate such a process. But at least one must undergo some austerity by giving up four unwanted principles, namely illicit sex, meat-eating, intoxication and gambling. Anyone can easily practice this tapasya, and then the chanting of the Hare Kṛṣṇa mantra will be effective without delay. One should not give up the process of austerity. If possible, one should bathe in the waters of the Ganges or Yamunā, or in the absence of the Ganges and Yamunā one may bathe in the water of the sea. This is an item of austerity. Our Kṛṣṇa consciousness movement has therefore established two very large centers, one in Vṛndāvana and another in Māyāpur, Navadvīpa. There one may bathe in the Ganges or Yamunā, chant the Hare Kṛṣṇa mantra and thus become perfect and return home, back to Godhead.