The Muṇḍaka Upanishad is one of the older, "primary" (mukhya) Upanishads commented upon by Adi Shankara. It is associated with the Atharvaveda. It figures as number 5 in the Muktika canon of 108 Upanishads.
This Upanishad divides all knowledge into two categories. The knowledge that leads to Self Realization is called Para Vidya or Divine Knowledge and everything else is called Apara Vidya or Knowledge of Material world. It is the first text to mention the six disciplines of Vedanga.
More Info: Mundaka Upanishad