Final answer:
The Magna Carta was the first known document to state that even kings had to follow laws. It codified the principles of due process and laid the foundation for the concept that all people, including kings, were subject to the law.
Step-by-step explanation:
The first known document to state that even kings had to follow laws is the Magna Carta. The Magna Carta was signed by King John of England in 1215. It codified the principles of due process, stating that a free man could not be penalized unless his peers had made a lawful judgment against him. This idea laid the foundation for the concept that all people, including kings, were subject to the law.