Constitution Article 1 Section 6

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

Senators shall be paid from the US Treasury and will escape arrest while in Attendance at the Session of their respective houses in all cases except for Treason.

No Senator or Representative shall, during the Time for which he was elected, be appointed to any civil Office under the Authority of the United States, which shall have been created, or the Emoluments whereof shall have been encreased during such time: and no Person holding any Office under the United States, shall be a Member of either House during his Continuance in Office.

 emolument = profit, salary or fees from office or employment

Representatives and senators can not any other office under the authority of the US, as well as no officer of the US shall be a senator or representative.