US Constitution - 27th Amendment

No law, varying the compensation for the services of the Senators and Representatives, shall take effect, until an election of Representatives shall have intervened.
The 27th Amendment is the most recent addition to the Constitution and was among the amendments that James Madison proposed in 1789. Ten of those amendments were approved and became known as the Bill of Rights, but this one languished for 203 years, making it the longest ratification process in U.S. history.