The Māṇḍūkya Upaniṣad is the shortest of all the Upanishads, and is assigned to Atharvaveda. It is listed as number 6 in the Muktikā canon of 108 Upanishads.
Mandukya Upanisad manuscript page, Verses 1–3, Atharvaveda (Sanskrit, Devanagari script)
The Upanishads are late Vedic and post-Vedic Sanskrit texts that "document the transition from the archaic ritualism of the Veda into new religious ideas and institutions" and the emergence of the central religious concepts of Hinduism. They are the most recent addition to the Vedas, the oldest scriptures of Hinduism, and deal with meditation, philosophy, consciousness, and ontological knowledge. Earlier parts of the Vedas dealt with mantras, benedictions, rituals, ceremonies, and sacrifices.
Image: Guru and Disciple I
Image: Yama teaches Nasiketha
Image: Tat tvam asi
Image: Vajrasuchi Upanishad sample i, Samaveda, Sanskrit, Devanagari script, 1728 CE manuscript