Visual representation not only of Muhammed but any living thing is generally prohibited.
We are not expected to produce human figures, in 2d or 3d - as you can see from human history, people tend to take the next step and create idols that they revere.
Historically this was reflected in belief systems like paganism or polytheism - however contemporaneously this is also seen - take the example of a fan who sees his favourite sports/movie/comedy/business etc personality as an "idol".