The Kali-Santāraṇa Upaniṣad (कलिसन्तरन) is a Vaishnava Vedantic text associated with the Krishna Yajurveda. It is the source of the "Hare Krishna" mantra, claiming that "These sixteen words are destructive of the evil effects of Kali Yuga." In the Muktikā canon of 108 Upanishads, it is counted as number 103.
The Upanishad asserts that whoever recites the mantra 35 million (sardha trikotir) times will be absolved from the sin of the murder of a Brahmin, the theft of gold, sexual intercourse with a woman of a lower caste, or even generally from any sin altogether. With a daily quota of 16 rosaries with 108 beads (1.5 hours of chanting), this count would be reached in 55.5 years.
The Upanishad at least predates the 16th century when the mantra was popularized by Chaitanya Mahaprabhu, and is believed to be of ancient origin by his followers (Gaudiya Vaishnavism).