Bṛhadāraṇyaka Upaniṣad ("Great Forest Teaching") 2.1.18

2.1.18

sa yatraitatsvapnyayā carati te hāsya lokāstaduteva mahārājo bhavatyuteva mahābrāhmaṇa utevoccāvacaṃ nigacchati sa yathā mahārājo jānapadāngṛhītvā sve janapade yathākāmaṃ parivartetaivamevaiṣa etatprāṇāngṛhītvā sveśarīre yathākāmaṃ parivartate

But when he moves about in sleep (and dream), then these are his worlds. He is, as it were, a great king; he is, as it were, a great Brâhmana; he rises, as it were, and he falls. And as a great king might keep in his own subjects, and move about, according to his pleasure, within his own domain, thus does that person (who is endowed with intelligence) keep in the various senses (prânas) and move about, according to his pleasure, within his own body (while dreaming).