You can download the full specification from http://www.iso.org/iso/home/store/catalogue_ics/catalogue_detail_ics.htm?csnumber=57853
for 238 Swiss Francs, and I'm sure that will have the answer. Otherwise, the best source I have is from "The C Programming Language" 2nd edition by K&R, section 4.6 on page 83 (emphasis added)
"The external static declaration is most often used for variables, but it can be applied to functions as well. Normally, function names are global, visible to any part of the entire program. If a function is declared static, however, its name is invisible outside of the file in which it is declared."
Note that the quote only refers to the declaration of the function, not its definition, although it's common practice for static functions, that the definition serves as the declaration as well.