It is clearly stated in the Kali Santarana Upanisad:

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

“After searching through all the Vedic literature, one cannot find a method of religion more sublime for this age than the chanting of the Hare Krishna mantra.”