The right to a trial by jury was included in the Magna Carta.
The Magna Carta is the English charter of rights approved by King John of England in 1215. This document has evolved constantly along the English history.
Its article 39 states that no men can be imprisoned, captured or kept from his liberties, outlawed, exiled or destroyed in any way, without the prior "lawful judgment of his peers or by the law of the land". The law of the land was the consuetudinary law, based on the customs.