What is here is also there; what is there, is also here. Who sees multiplicity but not the one indivisible Self must wander on and on from death to death.
The Kaha Upanishad is one of the older, "primary" Upanishads commented upon by Shankara. It is associated with the Taittiriya school of the Black Yajurveda. It figures as number 3 in the Muktika canon of 108 Upanishads.