Mandukya Karika, verse 2.18


निश्चितायां यथा रज्ज्वां विकल्पो विनिवर्तते ।
रज्जुरेवेति चाद्वैतं तद्वदात्मविनिश्चयः ॥ १८ ॥

niścitāyāṃ yathā rajjvāṃ vikalpo vinivartate |
rajjureveti cādvaitaṃ tadvadātmaviniścayaḥ || 18 ||

18. When the real nature of the rope is ascertained all illusions about it disappear and there arises the conviction that it is the one (unchanged) rope and nothing else; even so is the nature of the conviction regarding Ātman.

Shankara Bhashya (commentary)

When it is determined that it is nothing but the rope alone, then all illusions regarding the rope disappear and the (non-dual) knowledge that there exists nothing else but the rope, becomes firmly established. Similar is the knowledge,—like the light of the sun—produced by the negative Scriptural statements which deny all phenomenal attributes (in Ātmari),—statements like “Not this”, “Not this”, etc., leading to the knowledge of the real nature of Ātman, as: “All this is verily Ātman”, “(It is) without cause and effect, without internality and externality”, “(It is) ever without and within and beginningless”, “(It is) without decay and death, immortal, fearless, one and without a second.”