Answer:
Article 1, Section 2, Paragraph 3 of the Constitution of the United States reads:
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.
This section of the Constitution protects the right of each state to import persons (i.e. slaves) into their territory. It also sets a limit on the importation of slaves, requiring that any tax or duty imposed on such importation may not exceed $10 per person.