How do you deal with a husband who hits your toddler?

How do you deal with a husband who hits your toddler? My LO is just 3yo and my husband wants to discipline my kids using force. We've been arguing about this as I don't want my kids to be traumatized. Something happened just now while I was asleep. I want to file a complaint and have my son checked. He was slapped on the face and the marks are evident. Please advise me on what to do... Thank you.