What is the source of power to admit new states into the union?

the united states congress

The source of power for the United States Congress to admit new states into the Union is Article IV, Section 3, Clause 1, which states:

"New States may be admitted by the Congress into this Union; but no new State shall be formed or erected withing the Jurisdiction of any other State; nor any State formed by the Junction of two or more states, or Parts of States, without the Consent of the Legislatures of the States concerned as well as of the Congress."