Full Idea
We find that all mathematical knowledge has this peculiarity, that it must first exhibit its concept in intuition, and do so a priori, in an intuition that is not empirical but pure.
Gist of Idea
Mathematics can only start from an a priori intuition which is not empirical but pure
Source
Immanuel Kant (Prolegomena to Any Future Metaphysic [1781], 281)
Book Reference
Kant,Immanuel: 'Prolegomena to Any Future Metaphysic', ed/tr. Lucas,Peter G. [Manchester UP 1971], p.36
A Reaction
Later thinkers had grave doubts about this Kantian 'intuition', even if they though maths was known a priori. Personally I am increasing fan of rational intuition, even if I am not sure how to discern whether it is rational on any occasion.