Determine which amendment you are quoting.
Following your quotation marks, insert "U.S. Const." followed by "amend." and the Roman numeral of the amendment in parentheses as shown here: "Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof..."(U.S. Const. amend. I).
Use the same citation in your "Works Cited" page at the end of your paper.