Bṛhadāraṇyaka Upaniṣad ("Great Forest Teaching") 2.3.3

2.3.3

athāmūrtaṃ vāyuścāntarikṣaṃ caitadamṛtametadyadetattyantasyai tasyaitasyāmūrtasyaitasyāmṛtasyaitasya yata etasya tyasyaiṣa raso ya eṣa etasminmaṇḍale puruṣastyasya hyeṣa rasa ityadhidaivatam

But air and sky are immaterial, are immortal, are fluid, are indefinite. The essence of that which is immaterial, which is immortal, which is fluid, which is indefinite is the person in the disk of the sun, for he is the essence of tyad (the indefinite). So far with regard to the Devas.