Full Idea
For prime numbers we write (x not= 1 ∧ ∀u∀v(u x v = x → (u = 1 ∨ v = 1))). That is, the only way to multiply two numbers and a get a prime is if one of them is 1.
Gist of Idea
For primes we write (x not= 1 ∧ ∀u∀v(u x v = x → (u = 1 ∨ v = 1)))
Source
Peter Smith (Intro to Gödel's Theorems [2007], 04.5)
Book Reference
Smith,Peter: 'An Introduction to Gödel's Theorems' [CUP 2007], p.33