src/QueryBuilder/QueryBuilder.php
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 $this;
    }
public function fields($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;
    }
Set _source to search query.

@param mixed $fields
@return $this