Full Idea
If everything that a theory proves must be true, then it is a 'sound' theory.
Gist of Idea
If everything that a theory proves is true, then it is 'sound'
Source
Peter Smith (Intro to Gödel's Theorems [2007], 01.1)
Book Reference
Smith,Peter: 'An Introduction to Gödel's Theorems' [CUP 2007], p.2