For Kant mathematical objects are not pure objects of the understanding, although this view was later adopted by Marburg neo-Kantians, who rejected his separate faculty of sensibility after non-Euclidean geometries were discovered. They are objects attached to pure intuitions synthesized by productive imagination, which is the constructive aspect of sensibilty, in time for arithmetic, in space for geometry.
Correspondingly, Kant distinguishes symbolic and ostensive constructions. In other words, mathematical objects, while they are a priori, are like empirical objects in that they stand in the same relation to pure intuitions, as empirical objects stand to perceptions. Unlike a pure concept of the understanding, which only enables syntheses of possible intuitions which have to be supplied by sensibility, mathematical one "already contains a pure intuition in itself". This forces Kant to restrict mathematical objects to spatial and temporal magnitudes, because "qualities cannot be exhibited in anything but empirical intuition".
References are scattered throughout the Critique of Pure Reason, e.g. in the Preface to the second edition we find a famous quote:
"...new light flashed upon the mind of the first man (be he Thales or some other) who demonstrated the properties of the isosceles triangle. The true method, so he found, was not to inspect what he discerned either in the figure, or in the bare concept of it, and from this, as it were, read off its properties; but to bring out what was necessarily implied in the concepts that he had himself formed a priori, and had put into the figure in the construction by which he presented it to himself".
Elsewhere in the Critique and in the Prolegomena he describes establishing 7+5=12 by a priori synthesis, see Is number π empirical or a priori? But the central place for it is the section called Discipline of Pure Reason in its Dogmatic Use (SEP has a detailed article on it), where he writes that mathematical
"concepts must immediately be exhibited in concreto in pure intuition, through which anything unfounded and arbitrary instantly becomes obvious... to construct a concept means to exhibit a priori the intuition corresponding to it".