Your example seems to refer to an epigraph, which is a short passage normally used at the start of a book or chapter. There is no "single" answer. It depends entirely on the style guide or in-house style manual. The Chicago Manual of Style (13.36) says that
An author may wish to include an epigraph—a quotation that is
pertinent but not integral to the text—at the beginning of the
book. . . . The source of an epigraph is usually given on a line
following the quotation, sometimes preceded by a dash.
Although it does not specifically say that the "dash" used is an em dash, the common use of an en dash is for a number range or as a link between certain types of word pairs (at least in North America, if not the UK) which precludes it from being used in this context. (In the UK, it's more common for an en dash, with a space before and after, to be used instead of an em dash—but not, I think, in the specific case of an epigraph.)
Those textual examples presented by Chicago that use dashes all use em dashes—and there is no space between the dash and the epigraph's source attribution.
So, if a symbol is to be used, Chicago (at least implicitly) says it should be an em dash without a space. But other style guides might say something else—so, unfortunately, there is no definitive answer that can be given.