25 U.S.C. § 414

On and after August 25, 1937 , in all sales of tribal lands of the Choctaw and Chickasaw Indians in Oklahoma provided for by existing law, the Secretary of the Interior is hereby authorized to offer such lands for sale subject to a reservation of the mineral rights therein, including oil and gas, for the benefit of said Indians, whenever in his judgment the interests of the Indians will best be served thereby.