Source, in full
As reported by Charles Simmons in A Laconic Manual and Brief Remarker, containing over a thousand subjects alphabetically and systematically arranged (North Wrentham, Mass. 1852), p. 103. However, the original source of this statement is unknown.
Notes from Wikiquote
Cf. Joseph Addison in The Spectator No. 574 Friday, July 30, 1714, p. 655: In short, content is equivalent to wealth, and luxury to poverty; or, to give the thought a more agreeable turn, "content is natural wealth," says Socrates: to which I shall add, "luxury is artificial poverty.".