The phrase "obligatory reference" means a mention or citation that is necessary or required. It is something that must be included or acknowledged as per a standard or tradition. Full definition
UPDATE2: And now there's this from Senate GOP spokesman Scott Reif (with obligatory references to Gov. Andrew Cuomo, of course): (nystateofpolitics.com)
This flick is no exception, containing obligatory references and guest appearances from its fellow champions — some effective, some not. (kansascity.com)
Trosch's early paintings conjure a Florine Stettheimer fantasy of Park Avenue society ladies — Stettheimer being the one obligatory reference on the rare occasions when Trosch has been discussed in print - which made it a happy coincidence that his show partly coincided with the Jewish Museum's current retrospective of that also once — underrated painter. (fredericksfreisergallery.com)