hello friends I'm making an Insertion and get this errror

Access to undeclared static property: User :: $ rules

in which he points to the following line of code $ validator = Validator :: make ($ data = Input :: all (), User :: $ rules);

Model

<?php
 
class User extends \Eloquent {
 
public static $rules = array(
'email' => 'required'
 
);
public function grupos(){
return $this->belongsToMany('Group', 'users_groups', 'user_id', 'group_id');
}
 
}

Controller

public function store()
{
$validator = Validator::make($data = Input::all(), User::$rules);
 
if ($validator->fails())
{
return Redirect::back()->withErrors($validator)->withInput();
}
 
$user = Sentry::createUser(array(
'email' => Input::get('email'),
'password' => Input::get('password'),
'activated' => false,
'first_name' => Input::get('first_name'),
'last_name' => Input::get('last_name'),
'username' => Input::get('username'),
'cedula' => Input::get('cedula'),
));
 
$adminGroup = Sentry::findGroupById(2);
$user->addGroup($adminGroup);
 
return Redirect::route('users.index');
}