https://www.law.cornell.edu/uscode/text/18/4001
(a)No citizen shall be imprisoned or otherwise detained by the United States except pursuant to an Act of Congress.
Building on this theme, we now add a corroborating citation from the Federal Rules of Criminal Procedure, Rule 26, Notes of Advisory Committee on Rules, paragraph 2, in the middle, 2. TERRITORIAL JURISDICTION:
In order to define the jurisdiction of the Federal courts to conduct criminal prosecutions, one would have to find out what the specific definition of "Act of Congress," is. We find such a definition in Rule 54(c) of the Federal Rules of Criminal Procedure prior to Dec. 2002, wherein is defined "Act of Congress." Rule 54(c) states:
The Supreme Court rewrote the Federal Rules of Criminal Procedure and "conviently" omitted the above statement. You will need to find a copy dated before Dec. 1, 2002 that includes the above definition.