Which political document stated that no one, not even the king, was above the law?

The Magna Carta.

Explanation:

The Magna Carta was an agreement with the revolting English nobility that was sealed by King John Lackland at Runnymede, England on June 15, 1215. It is considered to be the main source of English constitutional law, as it guaranteed basic political freedoms of the nobility against the powers of the English king.

The charter curtailed the king's power and guaranteed greater political freedoms. The Church was protected from rule by the monarchs and was free to fill vacancies. Laws and judiciary were reformed and the behavior of the royal officials was brought under control.  

As for the case-law, free men could not be imprisoned or prosecuted without a judgment under the law, and merchants were allowed to roam the country freely and without restriction.