matrix-doc/event-schemas/examples/m.receipt

{
  "$ref": "core/room_edu.json",
  "type": "m.receipt",
  "content": {
    "$1435641916114394fHBLK:matrix.org": {
      "m.read": {
        "@rikj:jki.re": {
          "ts": 1436451550453
        }
      }
    }
  }
}