Error Message when required field not filled in

I want a Field Validation Error Message to appear when a required field is not filled in.  As an example, on the account page we have a required field called Business Team.  When this is not filled in, I want a error message to pop up rather than just the small "required" text that shows up next to the field.  What is the Field Validation to make the error message appear?

Thanks,

Judd