How do you handle it when someone throws you under the bus, trying to get you to take sides in an argument or issue?

When someone is having a disagreement that you are not in but they say ”add your name here agrees you were [_blank, tell him/her”_ (directed at you). Even if you thought they were right, you did not care to be involved but now you have to comment on what you were alledged to believe or said. When someone throws you under the bus like that, what is your reaction or what do you do about it?