"the words from one of the Chandogya-Upanishads"

The Chandogya Upanishad is an ancient source of principal fundamentals for Vedanta philosophy and is part of the Sama Veda, attributed to the singer of hymns, named Chandoga. It contains information on important spiritual practices like Dahara vidya, and Shandilya vidya.

Read a translation of the Chandogya Upanishad.

A detailed commentary is available on the Divine Life Society's website.