Bṛhadāraṇyaka Upaniṣad ("Great Forest Teaching") 2.3.2

2.3.2

tadetanmūrtaṃ yadanyadvāyoścāntarikṣāccaitanmartyametatsthitametatsattasyaitasya mūrtasyaitasya martyasyaitasya sthitasyaitasya sata eṣa raso ya eṣa tapati satohyeṣa rasaḥ

Everything except air and sky 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 sun that shines, for he is the essence of sat (the definite).