The requirement, such as it exists, is in 23.1585(a)(2). As to means of compliance, it should be referenced against the paragraph no. in FAA's AC23-8 but I don't have a copy to hand to confirm that (there's only so many documents one can keep copies of at home!).
If you haven't got a copy, but do have a fast connection or the patience, you can download 23-8 at
http://www2.airweb.faa.gov/Regulator...Highlight=23-8
The requirement is entirely a handling requirement - there are no additional structural requirements on part 23 aeroplanes which are a function of crosswind limit. It is also given as a
demonstrated limit, it is not a fixed operational requirement - although many operators not unreasonably elect to make it so.
G