The Taitttiriya Upanishad is one of the oldest Upanishads, compiled around 6th century BC. It is also called the “Black Yajurveda“, in contrast to the White Yajurveda that consists of the more well-known and organised Brihadaranyaka Upanishad and Isha Upanishad. The name refers to the sage Tittiri or a group of mythical students who became birds in order to gain knowledge. From the Taitttiriya Upanishad:
“Sight is indeed inseparable from the seer. As a mass of salt has neither inside nor outside but is altogether a mass of taste, thus, indeed, has the Self neither inside nor outside, but is altogether a mass of knowledge.”
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Taitttiriya Upanishad
We could find the earliest reference of Pancha Koshas of Five sheaths in Taittriya Upanishad.
Namaskaar, kindly advise list of translated books in hindi books.