“Experience, however, shows that neither a state nor a bank ever have had the unrestricted power of issuing paper money without abusing that power; in all states, therefore, the issue of paper money ought to be under some check and control; and none seems so proper for that purpose as that of subjecting the issuers of paper money to the obligation of paying their notes either in gold coin or bullion.”
— David Ricardo

“The Congress shall have Power…To coin Money, regulate the Value thereof, and of foreign Coin, and fix the Standard of Weights and Measures.”
— US Constitution Article I, Section 8, Clause 5

“No State shall…coin Money; emit Bills of Credit; make any Thing but gold and silver Coin a Tender in Payment of Debt.”
— US Constitution Article I, Section 10, Clause 1

This video gives a very good historical summary of US currency and its slow mutation from a Constitutional form of money which is gold or silver; to a fiat currency system which is simply paper and not backed by anything of real value.