Art. I, Sec. 8, Cl. 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
From Dumbauld's commentary:
As indicated by its genesis, the true interpretation of the "general welfare" clause is that it designates the purpose for which taxes may be imposed; however, it does not constitute a separate substantive power to provide for the general welfare by measures other than taxation.The "general welfare" clause originated in an attempt to made sure that payment of old debts incurred under the Confederation, as well as current expenses, was a legitimate occasion for exercise of the taxing power of the new government. Regardless of punctuation, the words "to provide for . . . the general welfare" do not constitute a substantive power but merely specify the purposes for which taxes may be imposed. The worlds were first used, in this connection, when speaking of assumption of state debts, and amounted to a declaration that those debts incurred for prosecution of the Revolutionary War were indeed incurred for the general welfare of the United States, and not merely for the local benefit of individual welfare of the particular debtor state. If the words were interpreted as a substantive power to legislate for the general welfare of the nation and people, the consequent enumeration of specific power granted in Article I, section 8, of the Constitution as adopted would be superfluous. Moreover, the federal government would then for all practical purposes be one of unlimited general legislative power, contrary to the general understanding of the time that it was one of enumerated and limited powers.


