Chandogya Upanishad.

The Chandogya Upanishad is one of the "primary" (mukhya) Upanishads. Together with the Jaiminiya Upanishad Brahmana and the Brihadaranyaka Upanishad it ranks among the oldest Upanishads, dating to the Vedic Brahmana period (probably before first millennium BCE).

1 Quotes

You could have golden treasure buried beneath you feet, and walk over it again and again, yet never find it because you don't realize it's there. Just so, all beings live every moment in the city of the Divine, but never find the Divine because it is hidden by the veil of illusion. -
— Chandogya Upanishad.