The strengthened finite Ramsey theorem:
For any positive integers n, k, m we can find N with the following property: if we color each of the n element subsets of S = {1, 2, 3,..., N} with one of k colors, then we can find a subset Y of S with at least m elements, such that all n element subsets of Y have the same color, and the number of elements of Y is at least the smallest element of Y.
The Paris–Harrington theorem states that the strengthened finite Ramsey theorem is not provable in Peano arithmetic. See the Wikipedia article on the Paris-HuntingtonHarrington theorem.