Full Idea
Most mathematical proofs, outside of set theory, do not explicitly state the set theory being employed.
Gist of Idea
Most mathematical proofs are using set theory, but without saying so
Source
Mark Colyvan (Introduction to the Philosophy of Mathematics [2012], 7.1.1)
Book Reference
Colyvan,Mark: 'An Introduction to the Philosophy of Mathematics' [CUP 2012], p.119