Article 1, Section 6, Clause 1
Articles of Confederation, art. 51 Mar. 1781
Freedom of speech and debate in Congress shall not be impeached or questioned in any Court, or place out of Congress, and the members of Congress shall be protected in their persons from arrests and imprisonments, during the time of their going to and from, and attendance on Congress, except for treason, felony, or breach of the peace.
The Founders' Constitution
Volume 2, Article 1, Section 6, Clause 1, Document 5
The University of Chicago Press
Easy to print version.