Subala Upanishad of Shukla-yajurveda, Chapter VIII

"The Ātmā of all which is immaculate, is located within the cave in the body. Ātmā which lives in the midst of the body filled with fat, flesh and phlegm in a seat very closely shut up with shining many-coloured walls resembling a Gandharva city and with the (subtle) essence going out of it (to other parts of the body), which seat may be likened to a plantain flower and is ever agitated like a water-bubble—this Ātmā is of an unthinkable form, the Divine Deva, associateless and pure, has tejas as its body, is of all forms, the Lord of all, the unthinkable and the bodiless, placed within the cave, immortal, shining, and bliss itself. He is a wise person who cognizes Ātmā thus, and not one who does not do so."