Article 1, Section 8

Congress shall have Power to exercise exclusive Legislation in all cases whatsoever over such District. (not exceeding ten Miles square) as my , by Cession of particular States, and the Acceptance of Congress, before the Seaat of the Government of the United States.

in reference to: http://www.law.umkc.edu/faculty/projects/ftrials/conlaw/hotissues.html (view on Google Sidewiki)