Bṛhadāraṇyaka Upaniṣad ("Great Forest Teaching") 2.3.4

2.3.4

athādhyātmamidameva mūrtaṃ yadanyatprāṇācca yaścāyamantarātmannākāśa etanmartyametatsthitametatsattasyaitasya mūrtasyaitasyamartyasyaitasya sthitasyaitasya sata eṣa raso yaccakṣuḥ sato hyeṣa rasaḥ

Now with regard to the body. Everything except the breath and the ether within the body is material, is mortal, is solid, is definite. The essence of that which is material, which is mortal, which is solid, which is definite is the Eye, for it is the essence of sat (the definite).