api-resources-users-controller.php

<?php

/**
 * Display a listing of the resource.
 *
 * @param User $user
 * @return \Illuminate\Http\Response
 */
public function index(User $user)
{
    return UsersResource::collection($user->with('posts')->paginate());
    // If you don't want to include the relationship in your response, don't use with()
    // return UsersResource::collection($user->paginate());
}