Full Idea
One thing Hume has taught us is that the necessity which causation involves is at most 'natural' or 'physical' necessity, not metaphysical necessity.
Clarification
'Natural' necessity follows laws of nature, 'metaphysical' necessity has to be true
Gist of Idea
Hume showed that causation could at most be natural necessity, never metaphysical necessity
Source
E.J. Lowe (A Survey of Metaphysics [2002], p.182)
Book Reference
Lowe,E.J.: 'A Survey of Metaphysics' [OUP 2002], p.182
A Reaction
Given Hume's epistemological scepticism, I don't think he would claim to have shown such a thing. See G.Strawson's book. Metaphysical necessity of causation is possible, but unknowable.