Which federalist papers define the 1st Amendment?

The First Amendment is primarily addressed in Federalist No. 10 and Federalist No. 51. In Federalist No. 10, James Madison discusses the dangers of faction and the importance of a large republic in mitigating those dangers, which indirectly relates to the protection of free expression. Federalist No. 51 emphasizes the need for checks and balances and the protection of individual rights, including freedom of speech and religion, as essential to preventing tyranny. Together, these papers lay the philosophical groundwork for the First Amendment's protections.