Article Six of the United States Constitution
In Martin v. Hunter's Lessee (1816), the United States Supreme Court ruled that it was more powerful than the state Supreme Courts.  In the case, the Virginia Supreme Court said that a United States Supreme Court decision was unconstitutional, and told all of the Virginia state courts not to obey it.  The United States Supreme Court ruled that it had the final say in legal issues that had to do with federal laws  not the state courts.  If a state court and the United States Supreme Court disagree, the Supreme Court is the supreme law of the land.