Constitutional FAQ Answer #4
Q4. "I was looking for what the Congress is prohibited from doing."
The Constitution doesn't so much say what they can't do as it does what they can. If you think about it, this restricts the Congress a lot more than if it spelled out what it is prohibited from doing. For info, see Article 1, Section 8; this is a list known as the "enumerated powers." However, the Commerce and Elastic clauses of that Section have ended up providing Congress with some relatively wide latitude in its application of power.
That having been said, Article 1, Section 9 does spell out some very specific things the Congress is prohibited from doing.
Though the intent of the Framers was to provide as little power as necessary to Congress, many felt that the lack of specificity would lead to power grabs. Within a few years of the ratification of the Constitution, these fears found their way into the Constitution in the form of the Bill of Rights. The first eight amendments restrict the powers of Congress much more specifically, explicitly, for example, restricting the power of Congress to regulate free speech.