Wikipedia says so:
http://en.wikipedia.org/wiki/Separation_of_church_and_state_in_the_United_States"The phrase separation of church and state is a common interpretation of the Establishment Clause of the First Amendment, which reads, "Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof . . . ." The phrase itself does not appear in any founding American document, but it has been quoted in opinions by the United States Supreme Court. (The first such mention was in Reynolds v. United States, 98 U.S. 145 in 1878."
The First Amendment:
"Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances."