On May 7, 1992, the 27th Amendment to our Constitution was ratified. The Amendment reads–
“No law, varying the compensation for the services of the Senators and Representatives, shall take effect, until an election of Representatives shall have intervened”.
Simply put, if Congressional members vote to increase (or decrease) their pay, any change wouldn’t go into effect until the next election. This restriction was intended to prevent Congress from granting itself a pay raise without the electorate’s approval.