In the Kali-santarana Upanisad it is stated:

hare krsna hare krsna
krsna krsna hare hare
hare rama hare rama
rama rama hare hare
iti sodasakam namnam
kali-kalmasa-nasanam
natah parataropayah
sarva-vedesu drsyate

“The sixteen words – Hare Krsna, Hare Krsna, Krsna Krsna, Hare Hare / Hare Rama, Hare Rama, Rama Rama, Hare Hare – are especially meant for counteracting the contaminations of Kali. To save oneself from the contamination of Kali, there is no alternative but the chanting of these sixteen words.”

From TheTeachings of Lord Caitanya by Srila Prabhupada