Adding source field to Elasticsearch query?

I have this query which is hitting the below table. How can I add source field to it to return specific columns from the _source field of this index?

GET mktg_percolate_postmetrics_mib/_search

{
  "query": {
    "bool": {
      "must": [
        {
          "range": {
            "end_date": {
              "gte": "now-30d",
              "lte": "now",
              "format": "epoch_millis"
            }
          }
        }
      ]
    }
  },
  "size": 0,
  "aggs": {
    "2": {
      "terms": {
        "field": "post_id",
        "size": 25,
        "order": {
          "1": "desc"
        }
      },
      "aggs": {
        "1": {
          "sum": {
            "script": "doc['facebook_likes'].value + doc['facebook_shares'].value + doc['facebook_comments'].value + doc['facebook_post_consumptions_by_type_link_clicks'].value",
            "lang": "expression"
          }
        }
      }
    }
  }
}