Soap or nope?

So please dont judge me and I probably should know the answer to this but are you supposed to or could you use soap on the inside of your butt? Like I've always used soap on the outside and water on the inside because I know that your really not supposed to use soap on the inside of your vagina and since your vag and butt are so close I didnt know if using soap was a bad thing or not. I know this is a little TMI but im gonna be honest and say that sweat is there lot and it doesnt smell the freshest. So soap or no soap?