Sway bar bolt!
Be sure to check them both because many of use including me were able to remove it by hand.
Just a tip for you is to use a allen wrench to hold the head of the bolt and a open end box wrench that is 17mm to turn the nut. I was able to do it with out the allen wrench but some have have the bolt spin on them when doing this. Buy some red loctite as well but before you do all this inspect the bolt and be sure it wasnt damaged or you will want to leave it alone and bring it in to the dealer.
If the threads are damaged you cant tighten the nut so look at it good and leave it loose if its messed up. My dealer wouldnt do anything since I fixed it myself and they just looked it over and said it was fine which I had figured any how. However my nut had only backed out about 1-2 turns and the others who had damaged bolts had the nut at the tip of the bolt which allowed the sway bar to rub on the threads.