I was surprised to see someone in our foursome stomp behind their ball before a chip shot. I didn't think you could do that, except on the tee box. I was only 90% confident in this, and I had just met the guy, so I didn't say anything.
It is literally Rule #1.1 - Play the course as you find it.
Rule #8.1 goes into more detail on this. Per Rule #8.1a, you are not allowed to move, bend, or break any living object.
The exception for the tee box is listed in Rule #8.2b.
