5 U.S.C. § 503 : US Code - Section 503: Witness fees and allowances

Search 5 U.S.C. § 503 : US Code - Section 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.