Article 1, Section 9, Clause 3
Delaware Declaration of Rights and Fundamental Rules11 Sept. 1776
11. That retrospective Laws, punishing Offenses committed before the Existence of such Laws, are oppressive and unjust and ought not to be made.
The Founders' Constitution
Volume 3, Article 1, Section 9, Clause 3, Document 3
The University of Chicago Press
Easy to print version.