Who document stated that the monarch's power was not absolute?

If you're talking about England then you could argue the Magna Carta of 1215 as it forced King Richard to place himself and members of his government under the rule of law. Also, look at the English Bill of Rights agreed to in the Glorious Revolution whereby William of Orange (King William III) and Mary II agreed to a Bill of Rights that laid out rights that all English citizens were entitled to and that the monarchy could not withhold.