The works of Ādi Śaṅkarācārya

Bṛhadāraṇyakopaniṣadbhāṣyam

Verse 143

योऽन्तरिक्षे तिष्ठन्नन्तरिक्षादन्तरो यमन्तरिक्षं न वेद यस्यान्तरिक्षं शरीरं योऽन्तरिक्षमन्तरो यमयत्येष त आत्मान्तर्याम्यमृतः ॥ ६ ॥

In plain words

The one who lives within the space between earth and sky, yet is other than that space, whom that space itself does not know, whose body that space is, who governs it from inside -- he is your self, the one who rules from within, and he does not die.

Close to the Sanskrit

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