Full Idea
It is one thing to believe something a priori and another for this belief to be epistemically justified. The latter is required for a priori knowledge.
Gist of Idea
A priori belief is not necessarily a priori justification, or a priori knowledge
Source
Paul Horwich (Stipulation, Meaning and Apriority [2000], §8)
Book Reference
'New Essays on the A Priori', ed/tr. Boghossian,P /Peacocke,C [OUP 2000], p.162
A Reaction
Personally I would agree with this, because I don't think anything should count as knowledge if it doesn't have supporting reasons, but fans of a priori knowledge presumably think that certain basic facts are just known. They are a priori justified.