Father-in-law smacked my son, what would you do in this situation?

[God Is Good]

My son is four and he came home tonight and told me that yesterday when he spend the night with my inlaws my father in law was driving and stopped the car, turned around and slapped his legs twice and gave him a "mean, crinkled face". I've never known him to be anything buy loving towards my son, but I obviously don't know what goes on when he's in their care. I asked him if he was doing something wrong and he said, "No, I was just holding the handle above my window".. he said he wasn't doing anything else. I'm not sure what he was doing for sure, and I'd like to know why he was smacked so I told him I would call and ask him about it. When I said that my husband said I was being stupid and didn't have any right to call him because he had given FIL permission to spank our son. He never consulted me about this and I was pissed and he said that it is his dad and he can give him permission to do anything he wants! One, what do I say to my FIL, if anything. Two, am I just being stupid? Do I not have a right to confront him since my husband said it was okay?

Btw, we do spank occasionally when need be (over clothes, on the butt) but I can't stand the thought of another person spanking my son! Am I overreacting?