From the Upanishads
न जायते म्रियते वा विपश्चित्
na jāyate mriyate vā vipaścit

“The knowing self is not born, and it does not die.”

Katha Upanishad 1.2.18
In plain terms

In the story, a teenager has asked Death directly what happens when a person dies, and this is the heart of the answer: the knowing part of you was never born, so dying cannot reach it.

It is the Katha Upanishad's boldest claim, and the Gita carries its spirit almost word for word in verse 2.20. Two texts, centuries apart, resting on the same still point.

All the verses, by the moment you need them