5 USC § 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.
(Pub. L. 89–554, Sept. 6, 1966, 80 Stat. 381.)