This clause of the United States Constitution is seen as both a temporary protection for the slave trade and as a clear indication that enslaved people were not considered United States citizens under the Constitution.
Section 9 : Powers Denied Congress
Clause 1 Migration or Importation
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.