You can pass it:
$user = User::find(1);
return Redirect::to('users/login')
->withMessage('Thanks for registering!');
->withUser($user);
And get it in your controller
$user = Session::get('user');
Or you could just pass the id
return Redirect::to('users/login')
->with('message', 'Thanks for registering!');
->with('user_id', $user->id);
And search for it again
$user = User::find(Session::get('user_id'));