The Hare Krsna mantra is specifically mentioned in many Upanisads, such as the Kali-santarana Upanisad, where it is said:
hare krsna hare krsna
krsna krsna hare hare
hare rama hare rama
rama rama hare hare
itisodasakam 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 Hare Krsna.”
From Srila Prabhupada’s purport to Sri Caitanya-caritamrta Adi-lila 3.40