Under Magna Carta the King cannot imprison a freeman without being convicted by a trial of his peers (1215)
Found in Magna Carta: A Commentary
Under King John’s reign, Clause 39 of Magna Carta became the foundation of the idea that a freeman could not be imprisoned without first being found guilty in a trial by his peers. This later became the idea behind the principle of “trial by jury”:
No freeman shall be taken or [and] imprisoned or disseised or exiled or in any way destroyed, nor will we go upon him nor send upon him, except by the lawful judgment of his peers or [and] by the law of the land.
Clause 39 guaranteeing the right of a freeman to a trial by his peers before he could be lawfully imprisoned is one of the most famous clauses in Magna Carta, along with the right to habeas corpus (that the accused must be presented to the court in person for charges to be read and the trial to begin). It became the foundation stone of English and then American court procedure and a symbol of “the traditional rights of Englishmen”. Some radical legal theorists like Lysander Spooner believed that not only did the jury in a trial have the right to pronounce on the guilt or innocence of the accused but also could rule on the very justice of the laws under which the defendant was accused, thus making a trial by jury into a quite radical tool for restricting the power of the state. The editor McKechnie had this to say about the phrase “per judicium parium” (by the judgement of his peers):
every judgment must be delivered by the accused man’s “equals.” The need for “a judgment of peers” was recognized at an early date in England. It was not originally a class privilege of the aristocracy, but a right shared by all grades of free–holders;  whatever their rank, they could not be tried by their inferiors. In this respect English custom did not differ from the procedure prescribed by feudal usage on the Continent of Europe. Two applications of this general principle had, however, special interest for the framers of Magna Carta: the “peers” of a Crown tenant were his fellow Crown tenants, who would normally deliver judgment in the Curia Regis; while the “peers” of the tenant of a mesne lord were the other suitors of the Court Baron of the manor. In either case, judgments were given per pares curiae. John, resorting wholesale to practices used sparingly in earlier reigns, had set these rules at defiance. His political and personal enemies were exiled, or deprived of their estates, by the judgment of a tribunal composed entirely of Crown nominees. Magna Carta promised a return to the ancient practice.