matrix-spec-proposals/event-schemas/schema/v1/m.room.message#m.text

{
    "$schema": "http://json-schema.org/draft-04/schema#",
    "type": "object",
    "title": "TextMessage",
    "description": "This message is the most basic message and is used to represent text.",
    "allOf": [{
        "$ref": "core#/definitions/room_event"
    }],
    "properties": {
        "content": {
            "type": "object",
            "properties": {
                "msgtype": {
                    "type": "string",
                    "enum": ["m.text"]
                },
                "body": {
                    "type": "string",
                    "description": "The body of the message."
                }
            },
            "required": ["msgtype", "body"]
        },
        "type": {
            "type": "string",
            "enum": ["m.room.message"]
        }
    }
}