Article 1, Section 10, Clause 2

Document 1

Articles of Confederation, art. 6

No state shall lay any imposts or duties, which may interfere with any stipulations in treaties, entered into by the united states in congress assembled, with any king, prince or state, in pursuance of any treaties already proposed by congress, to the courts of France and Spain.

The Founders' Constitution
Volume 3, Article 1, Section 10, Clause 2, Document 1
http://press-pubs.uchicago.edu/founders/documents/a1_10_2s1.html
The University of Chicago Press