Kaṭha Upaniṣad · 2.2.6
यथा सुधीः स्थाणुमयेषु निहितं नानुप्रविष्टं भवतीह शरीरे । गुणस्य संसारि ण चान्यथाऽतोऽसौ न शोचति कस्यचित् ॥ ६ ॥
yathā sudhīḥ sthāṇumayeṣu nihitaṃ nānupraviṣṭaṃ bhavatīha śarīre | guṇasya saṃsāri ṇa cānyathā'to'sau na śocati kasyacit
As the subtle ether is not contained in any place, so the Ātman, though dwelling in the body, is not mixed with it. Free from the wheel of saṃsāra, those who know this do not grieve for anyone.
The Ātman dwells in the body but does not confuse itself with it, just as space (ākāśa) penetrates all things without being contaminated. This essential non-differentiation between the witness and the witnessed liberates the sage from all affliction. One who understands that the inner Self is not affected by the body’s conditions, not even by the death of loved ones, transcends the wheel of saṃsāra and attains supreme peace.