The works of Ādi Śaṅkarācārya

Bṛhadāraṇyakopaniṣadbhāṣyam

Verse 138

न विज्ञातेर्विज्ञातारं विजानीयाः ।एषत आत्मा सर्वान्तरोऽतोऽन्यदार्तं ततोहोषस्तश्चाक्रायण उपरराम ॥ २ ॥

In plain words

You cannot turn the knower of knowing into something you know. This -- and only this -- is your self, the one living inside everything. Everything besides it carries its own suffering built in. Hearing this, Uṣasta fell silent.

Close to the Sanskrit

You cannot know the knower of knowing. This is your self, the one within all; everything other than this is subject to affliction. Then Uṣasta Cākrāyaṇa fell silent.