Article One of the United States Constitution
Section Nine reiterates the provision from Section Two that direct taxes must be apportioned by state populations.  This clause was also explicitly shielded from constitutional amendment prior to 1808 by Article V.  In 1913, the 16th Amendment exempted income taxes from this clause. Furthermore, no tax may be imposed on exports from any state. Congress may not, by revenue or commerce legislation, give preference to ports of one state over those of another; neither may it require ships from one state to pay duties in another. All funds belonging to the Treasury may not be withdrawn except according to law. Modern practice is that Congress annually passes a number of appropriations bills authorizing the expenditure of public money. The Constitution requires that a regular statement of such expenditures be published.