5 U.S.C. § 5537

(a) An employee as defined by section 2105 of this title (except an individual whose pay is disbursed by the Secretary of the Senate, the Chief Administrative Officer of the House of Representatives, or the Chief of the Capitol Police) or an individual employed by the government of the District of Columbia may not receive fees for service— (1) as a juror in a court of the United States or the District of Columbia; or (2) as a witness on behalf of the United States or the District of Columbia. (b) An official of a court of the United States or the District of Columbia may not receive witness fees for attendance before a court, commissioner, or magistrate judge where he is officiating. (c) For the purpose of this section, “court of the United States” has the meaning given it by section 451 of title 28 and includes the District Court of Guam and the District Court of the Virgin Islands.