Article I, Section 8, clause 2:

“To borrow Money on the credit of the United States.”

The Supreme Court has determined that that means Congress cannot neglect to pay the debts of the United States.

In Perry v. United States (1935), the Court cautioned that the power to borrow money is

a power vital to the government, upon which in an extremity its very life may depend. The binding quality of the promise of the United States is of the essence of the credit which is so pledged. Having this power to authorize the issue of definite obligations for the payment of money borrowed, the Congress has not been vested with authority to alter or destroy those obligations.