I need a legal citation, please

I need some help from the lawyers and legal-minded readers out there.

I understand the Supreme Court has ruled on more than one occasion that to exercise a constitutional right is not, in itself, grounds for suspicion.  For example, if someone wants to buy a gun, and they're asked all sorts of intrusive questions by local law enforcement (probably acting on instructions from NICS) about why they want it, that's out of line.  They're legally entitled to buy it, so why the questions?

Unfortunately, law enforcement is often notoriously heavy-handed in cases like this.  I want to help the person concerned (one of my students) by being able to cite that Supreme Court ruling.  (If there are other rulings along the same lines, either from lower levels of federal courts or from state courts, those would be useful, too.)

If you can help, please leave a comment with that information.  Thanks!

Peter