No citation is needed. That is basic civics 101. A federal official gets their authority from laws enacted by Congress and have supremacy clause authority from the federal Constitution. A sheriff gets his authority from the respective state constitution and/or state statute. Federal officers do not answer or report to state officials in the faithful performance of their duties. For example, ever wonder why a state court cannot give an order to a federal agency such as the EPA. It is because the federal government is considered a separate sovereign from the state.
I think the federal government has much overstepped its federal Constitution limit of powers but in all technicality unto federal laws are ruled unconstitutional then the federal agents have power even over local sheriffs to enforce and carry out federal law.
Opinion: "The Framers rejected the concept of a central government that would act upon and through the States, and instead designed a system in which the State and Federal Governments would exercise concurrent authority over the people. The Federal Government's power would be augmented immeasurably and impermissibly if it were able to impress into its serviceand at no cost to itselfthe police officers of the 50 States" ... "Federal control of state officers would also have an effect upon the separation and equilibration of powers between the three branches of the Federal Government itself."
Tarheel25
Read the links and start learning that the feds do not have authority over sheriffs.
http://www.criminalgovernment.com/docs/sheriff/sher_boots_feds.html
http://www.mintpressnews.com/sheriffs-defy-feds-refusing-honor-detainer-requests/192541/
http://www.fourwinds10.net/siterun_data/government/us_constitution/news.php?q=1321294429
http://blog.tenthamendmentcenter.com/2012/04/its-sheriffs-first-in-arizona/