US Constitution and Amendments
Section 2, Clause 3
Representatives and direct taxes shall be apportioned among the several States which may be included within this Union, according to their respective Numbers.
Section 8, Clause 1
The Congress shall have Power to lay and collect Taxes, Duties, Imposts and Excises, to pay the Debts and provide for the common Defence and general Welfare of the United States; but all Duties, Imposts and Excises shall be uniform throughout the United States.
Section 9, Clause 4
No Capitation, or other direct, Tax shall be laid, unless in proportion to the Census or Enumeration herein before directed to be taken.
Section 9, Clause 5
No Tax or Duty shall be laid on Articles exported from any State.