Full Idea
If a proposition is thought along with its necessity, it is an a priori judgement; if it is, moreover, also not derived from any proposition except one that in turn is valid as a necessary proposition, then it is absolutely a priori.
Gist of Idea
Propositions involving necessity are a priori, and pure a priori if they only derive from other necessities
Source
Immanuel Kant (Critique of Pure Reason [1781], B003)
Book Reference
Kant,Immanuel: 'Critique of Pure Reason', ed/tr. Guyer,P /Wood,A W [CUO 1998], p.137
A Reaction
The misunderstanding behind this is that we can obtain certainty in this way. I presume that consistency with empirical experience would increase our certainty of (say) maths or logic. There is no 'pure' a priori, delivering 'pure' necessity.