He was proud of having discovered a new faculty in man -- the faculty of making synthetic a priori judgments ... It is the ambition and rivalry of all the younger philosophers to discover "new faculties"! ... How are synthetic a priori judgements possible, Kant asked himself. And what was actually his answer? By virtue of virtue -- but unfortunately not in five words but so complicatedly, respectably, with such a show of German profundity and sinuosity, that one failed to hear the funny German simple-mindedness inherent in such an answer.
Beyond Good and Evil, trans. by Marianne Cowan, section 11. (I have no idea how to cite this.)
Unfortunately I haven't read enough Kant (I've only read the Grounding for the Metaphysics of Morals), but Nietzsche's criticism does strike me as true.