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
(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.