Article 1, Section 9, Clause 1
St. George Tucker, Blackstone's Commentaries 1:App. 2901803
This article, at the time the constitution was framed, was deemed necessary to prevent an opposition, on that ground, to it's adoption in those states which still permitted the importation of slaves from Africa, and other foreign parts. A more liberal policy has since prevailed, so far as to render it probable that congress will never have occasion to exert the right of prohibiting the importation of slaves, such importation being now prohibited by the laws of all the states in the union. But should any of them shew an inclination to rescind the present prohibitions, congress, after the year 1808, will be able to interpose it's authority to prevent it, and impose some partial restraint upon the farther extension of the miseries of mankind. How to remove the calamities of slavery from among us, is left to the wisdom of the state government; the federal government can only prevent the further importation of slaves after the period limited.
The Founders' Constitution
Volume 3, Article 1, Section 9, Clause 1, Document 18
The University of Chicago Press
Tucker, St. George. Blackstone's Commentaries: With Notes of Reference to the Constitution and Laws of the Federal Government of the United States and of the Commonwealth of Virginia. 5 vols. Philadelphia, 1803. Reprint. South Hackensack, N.J.: Rothman Reprints, 1969.
Easy to print version.