Today's selection come's from the Constitution.  The First Amendment specifically:

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.

For something that is invoked a great many times, it is startling how many people actually have no idea what the First Amendment actually says.  Specifically in the area of church and state.  I have come across numerous people over the years that have said, "the words 'separation of church and state' are not anywhere in the Constitution."   To this I immediately point out the first clause of the First Amendment. 

Congress can not make any law establishing religion.  I am pretty sure that equates to a separation of church and state.  The Founders got a first hand view of what religious persecution conducted by the colonial governments looked liked.  They made sure we never do.  So the next time you hear someone argue there is no legal requirement for the separation of religion and government please correct them.  Politely or otherwise.