dvāparīyair janair viṣṇuḥ pañcarātrais tu kevalaiḥ kalau tu nāma-mātreṇa pūjyate bhagavān hariḥ

dvapariyair janair visnuh pancaratrais tu kevalaih
kalau tu nama-matrena pujyate bhagavan harih
"In the Dvapara-yuga people should worship Lord Visnu only by the regulative principles of the Narada-pancaratra and other such authorized books. In the age of Kali, however, people should simply chant the holy names of the Supreme Personality of Godhead." The Hare Krsna mantra is specifically mentioned in many Upanisads, such as the Kali-santarana Upanisad, where it is said:
 

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