The idea that the Constitution is the supreme law of the land is found only in one article of the Constitution. It is found in Article VI. Article VI is, in general, concerned with the powers of the federal government.
In Article VI, we are told that the Constitution, as well as any laws that are created by Congress in accordance with the Constitution, will be the supreme law of the land. The exact words of Article VI are
This Constitution, and the Laws of the United States which shall be made in Pursuance thereof; and all Treaties made, or which shall be made, under the Authority of the United States, shall be the supreme Law of the Land...
So, the answer to your question is that it is Article VI that states that the Constitution is the supreme law of the land.