[jQuery] jQuery validation - Numbers not allowed

[nouky]

I have this code:
$(document).ready(function() {
        // validate signup form on keyup and submit
        var validator = $("#signupform").validate({
                rules: {
                        firstname: "required",
                        lastname: "required",
                        username: {
                                required: true,
                                minlength: 2,
                                remote: "users.php"
                        }
                }
        });

});

How do I set the validation that no number are allowed in the
firstname text box????