Full Idea
Mathematical propositions are always a priori judgments and are never empirical, because they carry necessity with them, which cannot be derived from experience.
Gist of Idea
Maths must be a priori because it is necessary, and that cannot be derived from experience
Source
Immanuel Kant (Critique of Pure Reason [1781], B014)
Book Reference
Kant,Immanuel: 'Critique of Pure Reason', ed/tr. Guyer,P /Wood,A W [CUO 1998], p.144
A Reaction
Personally I like the idea that maths is the 'science of patterns', but then I take it that the features of patterns will be common to all possible worlds. Presumably a proposition could be contingent, and yet true in all possible worlds.