The works of Ādi Śaṅkarācārya

Bṛhadāraṇyakopaniṣadbhāṣyam

Verse 159

यो विज्ञाने तिष्ठन्विज्ञानादन्तरो यंविज्ञानं न वेद यस्य विज्ञानँ शरीरं योविज्ञानमन्तरो यमयत्येष त आत्मान्त-र्याम्यमृतः ॥ २२ ॥

In plain words

The one who lives inside your own understanding, yet is nothing it can claim as its own, whom understanding itself never knows, whose body it is, who governs it from within -- that is your self, the ruler seated inside everything, and death does not touch him.

Close to the Sanskrit

He who, dwelling in understanding, is other than understanding, whom understanding does not know, whose body understanding is, who controls understanding from within -- he is your self, the inner controller, immortal.