Article 4, Section 3, Clause 1
Document 5
Charles Pinckney, Observations on the Plan of Government
1787Farrand 3:119--20The article impowering the United States to admit new States into the Confederacy is become indispensable, from the separation of certain districts from the original States, and the increasing population and consequence of the Western Territory. I have also added an article authorizing the United States, upon petition from the majority of the citizens of any State, or Convention authorized for that purpose, and of the Legislature of the State to which they wish to be annexed, or of the States among which they are willing to be divided, to consent to such junction or division, on the terms mentioned in the article.--The inequality of the Federal Members, and the number of small States, is one of the greatest defects of our Union. It is to be hoped this inconvenience will, in time, correct itself; and, that the smaller States, being fatigued with the expence of their State Systems, and mortified at their want of importance, will be inclined to participate in the benefits of the larger, by being annexed to and becoming a part of their Governments. I am informed sentiments of this kind already prevail; and, in order to encourage propositions so generally beneficial, a power should be vested in the Union, to accede to them whenever they are made.
The Founders' Constitution
Volume 4, Article 4, Section 3, Clause 1, Document 5
http://press-pubs.uchicago.edu/founders/documents/a4_3_1s5.html
The University of Chicago Press
Farrand, Max, ed. The Records of the Federal Convention of 1787. Rev. ed. 4 vols. New Haven and London: Yale University Press, 1937.