Cannot get $user from route in controller method with Laravel

For which possible reason dd(User::find(123)->id) returns the expected user id while dd($user->id) returns null when opening http://backend.test/my-route/123 ?

routes/api.php:

Route::get('my-route/{user}', 'My\Path\To\Controllers\MyController@myMethod');

My/Path/To/Controllers/MyController.php:

    public function myMethod(User $user)
    {
        dd(User::find(123)->id);
        dd($user->id);
    }

via Chebli Mohamed