Federalist Papers
Article I      Section 9      clause 1
The Migration or Importation of such Persons as any of the States now existing shall think proper to admit, shall not be prohibited by the Congress prior to the Year one thousand eight hundred and eight, but a Tax or duty may be imposed on such Importation, not exceeding ten dollars for each Person.

FEDERALIST NO. 54     Paragraph: 4
FEDERALIST NO. 42     Paragraph: 5