Article 1, Section 6, Clause 1

Document 8

Vermont Constitution of 1786, CH. 1, ART. 16

The freedom of deliberation, speech, and debate, in the legislature, is so essential to the rights of the people, that it cannot be the foundation of any accusation or prosecution, action or complaint, in any other court or place whatsoever.

The Founders' Constitution
Volume 2, Article 1, Section 6, Clause 1, Document 8
http://press-pubs.uchicago.edu/founders/documents/a1_6_1s8.html
The University of Chicago Press

Thorpe, Francis Newton, ed. The Federal and State Constitutions, Colonial Charters, and Other Organic Laws of the States, Territories, and Colonies Now or Heretofore Forming the United States of America. 7 vols. Washington, D.C.: Government Printing Office, 1909.