U.S. Constitution - Article 1 Section 6
Article 1 - The Legislative Branch
Section 6 - Compensation
(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.) (The preceding words in parentheses were modified by the 27th
Amendment.) 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.
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 increased 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.