Paragraph 4 of the 14th Amendment to the U.S. Constitution states in full (emphasis added):
4. The validity of the public debt of the United States, authorized by law, including debts incurred for payment of pensions and bounties for services in suppressing insurrection or rebellion, shall not be questioned. But neither the United States nor any State shall assume or pay any debt or obligation incurred in aid of insurrection or rebellion against the United States, or any claim for the loss or emancipation of any slave; but all such debts, obligations and claims shall be held illegal and void.
Update: Bernie Sanders explains in detail for those who don't get it. (H/T to the LW )