BṚHADĀRAṆYAKA UPANISHAD · 4.3.18

तद् यथा महामत्स्य उभे कूले अनुसंचरति पूर्वं चापरं च ।
एवम् एवायं पुरुष एताव् उभाव् अन्ताव् अनुसंचरति स्वप्नान्तं च बुद्धान्तं च ॥

tad yathā mahāmatsya ubhe kūle anusaṃcarati pūrvaṃ cāparaṃ ca ।
evam evāyaṃ puruṣa etāv ubhāv antāv anusaṃcarati svapnāntaṃ ca buddhāntaṃ ca ॥

'In fact, as a large fish moves along the two banks of a river, the right and the left, so does that person move along these two states, the state of sleeping and the state of waking.