Taittiriyaka-Upanishad: Second Valli, Fifth Anuvaka
“Understanding performs the sacrifice, it performs all sacred acts. All Devas worship understanding as Brahman, as the oldest. If a man knows understanding as Brahman, and if he does not swerve from it, he leaves all evils behind in the body, and attains all his wishes.” The embodied Self of this (consisting of understanding) is the same as that of the former (consisting of mind).
Different from this, which consists of understanding, is the other inner Self, which consists of bliss. The former is filled by this. It also has the shape of man. Like the human shape of the former is the human shape of the latter. joy is its head. Satisfaction its right arm. Great satisfaction is its left arm. Bliss is its trunk. Brahman is the seat (the support).
On this there is also the following Sloka: