Where in the Consitution does it say this. The Constution, in Art 1 Sec 8, gives Congress the power to:
"To coin Money, regulate the Value thereof and of foreign Coin, and fix the standards of Weights and Measures".
I can find nothing fixing the number of dollars to the grain of gold, or any other metal. (Silver was also used for coins)
This is what I was thinking of when I wrote what I did - The Coinage Act of 1792: Congress defined the dollar as being 371.25 grains of silver. It then regulated the value of gold coins at 24.75 grains. This means you could exchange 15 grains of silver for every grain of gold.
Nevertheless, the Constitution didn't give Congress the power to delegate its authority to define the dollar and regulate its value to a third party bank otherwise known as the Federal Reserve.