Amendment IV
The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no warrants shall issue, but upon probable cause, supported by oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized.
It’s a good thing Madison didn’t rule out government snooping in addition to searches and seizures. Otherwise the government would be in violation of the Fourth.