code: public function fields($fields = false)
      {
          if ($fields === false) {
              $this->query['body']['_source'] = false;
          } elseif ((array)$fields == ['*']) {
              unset($this->query['body']['_source']);
          } else {
              $this->query['body']['_source'] = $fields;
          }
          return $this;
      }
docs: Set _source to search query.
  
  @param mixed $fields
  @return $this