Dhaththathreya Upanishad

It is attached to the Atharvaveda, and classified as a text of the Vaishnava sect, which worships the god Vishnu. The text is a Tantra and Vaishnava work, likely one of the relatively recent, 14th- or 15th-century CE era composition compared to other Upanishads. The text presents a Vaishnava mantra that is the most popular mantra in Dattatreya tradition, as well as a series of tantric mantras for the worship of sage Dattatreya, a form of Vishnu. The text asserts that the worship of Vishnu, Narayana and Dattatreya leads one to the nature of Truth Bliss-Knowledge.