The Constitution – Article I, section 6, clause 1

Compensation Clause

“The Senators and Representatives shall receive a Compensation for their Services, to be ascertained by Law, and paid out of the Treasury of the United States.”

Privilege from Arrest

“They shall in all Cases, except Treason, Felony and Breach of the Peace, be privileged from Arrest during their Attendance at the Session of their respective Houses, and in going to and returning from the same…”

Speech and Debate Clause

“…and for any Speech or Debate in either House, they shall not be questioned in any other Place.”