Full Idea
Some of our ideas have a natural correspondence and connexion one with another. ...Besides this there is another connexion of ideas wholly owing to chance or custom.
Gist of Idea
Some ideas connect together naturally, while others connect by chance or custom
Source
John Locke (Essay Conc Human Understanding (2nd Ed) [1694], 2.33.05)
Book Reference
Locke,John: 'Essay Concerning Human Understanding', ed/tr. Nidditch,P.H. [OUP 1979], p.395
A Reaction
This strikes me as a more promising account of associations that the one offered by Hume, since Locke distinguishes the associations that seem somehow right and natural from those that seem merely conventional.