Full Idea
The original Gödel construction gives us a sentence that a theory shows is true if and only if it satisfies the condition of being unprovable-in-that-theory.
Gist of Idea
There is a sentence which a theory can show is true iff it is unprovable
Source
report of Kurt Gödel (On Formally Undecidable Propositions [1931]) by Peter Smith - Intro to Gödel's Theorems 20.5
Book Reference
Smith,Peter: 'An Introduction to Gödel's Theorems' [CUP 2007], p.173