Article 4, Section 3, Clause 1
Articles of Confederation, art. 111 Mar. 1781
Article XI. Canada acceding to this confederation, and joining in the measures of the united states, shall be admitted into, and entitled to all the advantages of this union: but no other colony shall be admitted into the same, unless such admission be agreed to by nine states.
The Founders' Constitution
Volume 4, Article 4, Section 3, Clause 1, Document 2
The University of Chicago Press
Easy to print version.