Salaries of Members of Congress: Recent Actions and Historical Tables [Updated January 13, 2020] [open pdf - 1MB]
From the Summary: "Congress is required by Article I, Section 6, of the Constitution to determine its own pay. In the past, Congress periodically enacted specific legislation to alter its pay; the last time this occurred affected pay in 1991. More recently, pay has been determined pursuant to laws establishing formulas for automatic adjustments."
CRS Report for Congress, 97-1011
Congressional Research Service: https://crsreports.congress.gov/