Amendment XVI to the Constitution of the United States
“The Congress shall have power to lay and collect taxes on
incomes, from whatever source derived, without apportionment
among the several States, an without regard to any census
or enumeration.”
Yep, but they still must be uniform throughout the United States. You can’t charge millionaires in one city at one tax rate, and differently in another place.
See this regarding “apportionment”: http://www.utexas.edu/law/faculty/calvinjohnson/directtax.pdf