Article I, § 9, clause 1, of the Constitution. This clause provides: "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....”
The specific limit on congressional power before 1808 can be construed to state that after 1808, Congress would have power over immigration and importation of people.
In 1803, Congress passed a bill that would become effective in 1808, which prohibited bringing to the country “Negro and mulatto” immigrants. The law applied to black freemen, not to black slaves.
Isn't it interesting that it did not apply to slaves, only free blacks and was effective after the date in Article I, § 9, clause 1, of the Constitution?
https://wlr.law.wisc.edu/wp-content/uploads/sites/1263/2023/12/Geoffrey-Heeren-Immigration-Law-and-Slavery-Rethinking-the-Migration-or-Importation-Clause-2023-Wis.-L.-Rev.-1125.pdf