Article I, Section 6, Clause 1:
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.
This clause is practically obsolete. It applies only to arrests in civil suits, which were still common in this country at the time the Constitution was adopted. 1 It does not apply to service of process in either civil 2 or criminal cases. 3 Nor does it apply to arrest in any criminal case. The phrase "treason, felony or breach of the peace" is interpreted to withdraw all criminal offenses from the operation of the privilege. 4