The balance of federal powers and those powers held by the states as defined in the Supremacy Clause of the U.S. Constitution was first addressed in the case of McCulloch v. Maryland (1819). The Court's decision by Chief Justice John Marshall asserted that the laws adopted by the federal government, when exercising its constitutional powers, are generally paramount over any conflicting laws adopted by state governments. After McCulloch, the primary legal issues in this area concerned the scope of Congress' constitutional powers, and whether the states possess certain powers to the exclusion of the federal government, even if the Constitution does not explicitly limit them to the states.

Based on this passage, where does the power reserved for the states constitutionally originate from? Let's give stream of consciousness first:
The relevant information to answer the above question is: The balance of federal powers and those powers held by the states as defined in the Supremacy Clause of the U.S. Constitution was first addressed in the case of McCulloch v. Maryland (1819).
So, the final answer is the Supremacy Clause of the U.S. Constitution.