Kali Santarana Upanisad & Caitanya Upani - The modern era Kali-Santarana Upanishad is the earliest known Hindu text where this widely known mantra appears. It was popularized by one of the Bhakti movement leaders Chaitanya Mahaprabhu in the 16th century. The Maha-mantra enunciated in this Upanishad Hare krsna hare krsna krsna krsna hare hare I hare rama hare ráma rama rama hare Hare || These sixteen names will destroy the contaminations of Kaliyuga.
 
One cannot see any other better method in all the Vedas Kali santarana Upanisad namany.astavasta co-sobhanani tani nityam ye japanti dhiras te vai mayámatitaranti nanyah paramam mantra parama-rahasyam nityam ávartayati Those who chant these sixteen glorious names completely surpass-mäyä. Others do not. Even the nitya-siddhas chant-this supremely secret mantra at all times. -Caitanya Upanisad