Article 2, Section 2, Clauses 2 and 3
Thomas Jefferson to William Dunbar17 July 1803Works 10:19
Before you receive this, you will have heard, through the channel of the public papers, of the cession of Louisiana by France to the United States. The terms as stated in the National Intelligencer, are accurate. That the treaty may be ratified in time, I have found it necessary to convene Congress on the 17th of October; and it is very important for the happiness of the country that they should possess all the information which can be obtained respecting it, that they make the best arrangement practicable for its good government. It is the most necessary, because, they will be obliged to ask from the People an amendment of the Constitution, authorizing their receiving the province into the Union, and providing for its government; and the limitations of power which shall be given by that amendment, will be unalterable but by the same authority.
The Founders' Constitution
Volume 4, Article 2, Section 2, Clauses 2 and 3, Document 26
The University of Chicago Press
The Works of Thomas Jefferson. Collected and edited by Paul Leicester Ford. Federal Edition. 12 vols. New York and London: G. P. Putnam's Sons, 1904--5.
Easy to print version.