Article 1, Section 9, Clause 6

No Preference shall be given by any Regulation of Commerce or Revenue to the Ports of one State over those of another; nor shall Vessels bound to, or from, one State, be obliged to enter, clear or pay Duties in another.

1.Records of the Federal Convention
2.Luther Martin, Maryland House of Delegates, 29 Nov. 1787
3.Luther Martin, Genuine Information, 1788
4.James Iredell, Proposed Amendment to the Constitution, North Carolina Ratifying Convention, 1 Aug. 1788