Full Idea
The very concept of a cause obviously contains the concept of a necessity of connection with an effect and a strict universality of rule, which would be entirely lost if one sought, as Hume did, to derive it from a frequent association.
Gist of Idea
Causation obviously involves necessity, so it cannot just be frequent association
Source
Immanuel Kant (Critique of Pure Reason [1781], B005)
Book Reference
Kant,Immanuel: 'Critique of Pure Reason', ed/tr. Guyer,P /Wood,A W [CUO 1998], p.138
A Reaction
It is not clear (in the next paragraphs) whether Kant is saying causation is necessary because it is knowable a priori, or knowable a priori because it is necessary. I am quite sure that Hume cannot be dismissed as glibly as this.