Mandukya Karika, verse 3.46
यदा न लीयते चित्तं न च विक्षिप्यते पुनः ।
अनिङ्गनमनाभासं निष्पन्नं ब्रह्म तत्तदा ॥ ४६ ॥
yadā na līyate cittaṃ na ca vikṣipyate punaḥ |
aniṅganamanābhāsaṃ niṣpannaṃ brahma tattadā || 46 ||
46. When the mind does not merge in the inactivity of oblivion, or become distracted by desires, that is to say, when the mind becomes quiescent and does not give rise to appearances, it verily becomes Brahman.
Shankara Bhashya (commentary)
When the mind brought under discipline by the above-mentioned1 methods, does not fall into the oblivion of deep sleep, nor is distracted by external objects, that is to say, when the mind becomes quiescent2 like the flame of a light kept in a windless place; or when3 the mind does not appear in the form of an object,—when the mind is endowed with these characteristics, it verily becomes one4 with Brahman.