5 U.S.C. § 503: Witness fees and allowances

(a) For the purpose of this section, “agency” has the meaning given it by section 5721 of this title. (b) A witness is entitled to the fees and allowances allowed by statute for witnesses in the courts of the United States when—(1) he is subpenaed under section 304(a) of this title; or (2) he is subpenaed to and appears at a hearing before an agency authorized by law to hold hearings and subpena witnesses to attend the hearings.