Article 1, Section 8, Clause 3 (Commerce)

Document 19

James Madison to Joseph C. Cabell

For a like reason, I made no reference to the "power to regulate commerce among the several States." I always foresaw that difficulties might be started in relation to that power which could not be fully explained without recurring to views of it, which, however just, might give birth to specious though unsound objections. Being in the same terms with the power over foreign commerce, the same extent, if taken literally, would belong to it. Yet it is very certain that it grew out of the abuse of the power by the importing States in taxing the non-importing, and was intended as a negative and preventive provision against injustice among the States themselves, rather than as a power to be used for the positive purposes of the General Government, in which alone, however, the remedial power could be lodged.

The Founders' Constitution
Volume 2, Article 1, Section 8, Clause 3 (Commerce), Document 19
http://press-pubs.uchicago.edu/founders/documents/a1_8_3_commerces19.html
The University of Chicago Press

Letters and Other Writings of James Madison. Published by order of Congress. 4 vols. Philadelphia: J. B. Lippincott & Co., 1865.

Easy to print version.