I migrated a application from Laravel 4 to Laravel 5.1.
In Laravel 4 , we have User eloquent class in app/models folder.
So I keep it in app/Models in Laravel 5.1 verision. And set  App\Models namespace.
 When I use Laravel default authentication method.
I faced the following problem:

FatalErrorException in EloquentUserProvider.php line 126:Class '\App\User' not found


To solve that error, we need to customize authentication model for application.

Edit in config/auth.php.
Change 
'model' => App\User::class,
to your customize class
In my situation :
'model' => App\Models\User::class,