Mandukya Karika, verse 3.30

Text

अद्वयं च द्वयाभासं मनः स्वप्ने न संशयः ।
अद्वयं च द्वयाभासं तथा जाग्रन्न संशयः ॥ ३० ॥

advayaṃ ca dvayābhāsaṃ manaḥ svapne na saṃśayaḥ |
advayaṃ ca dvayābhāsaṃ tathā jāgranna saṃśayaḥ || 30 ||

30. There is no doubt that the mind, which is, in fact, non-dual appears as dual in dream; in like manner undoubtedly that which is non-dual, appears as dual in the waking state also.

Shankara Bhashya (commentary)

Really speaking, the snake is identical with the rope. In like manner, the mind which is nondual1 as Ātman appears undoubtedly in dual forms in dreams. Verily in dream, such objects of perception as elephants, etc., or their perceivers such as eyes, etc., have2 no existence independently of consciousness (mind). Similar3 is the case in the waking state as well. For (conciousness) mind, which is the highest Reality, is common to both.