Full Idea
Hume held that constant conjunction between As and Bs is both necessary and sufficient for a causal relation. If As and Bs are conjoined that is sufficient for a causal relation; if A and B are causally related, necessarily they are constantly conjoined.
Gist of Idea
For Hume a constant conjunction is both necessary and sufficient for causation
Source
report of David Hume (Treatise of Human Nature [1739]) by Tim Crane - Causation 1.2.2
Book Reference
'Philosophy: a Guide Through the Subject', ed/tr. Grayling,A.C. [OUP 1995], p.186
A Reaction
A helpful connection between Hume and the modern debate about conditions for causation (e.g. Mackie). It sounds as if, to spot the necessary condition, you need to independently see that A and B are causally related, which regularity does not allow.