Which lens for boxing?

I've been asked to shoot a night of amateur boxing matches and want to take the sharpest photos possible. The quality of my work can lead me to landing a contract to do all of the photography for this promoter. I will be ringside for each bout, but I've never done any low light sports photography like this and I need some help. I'm thinking of using prime lenses, but not sure as to which focal length would be best. Any suggestions? Thanks in advance.