Whom does macbeth have murdered to keep one of the witches' prophecies from coming true

His "best friend" and fellow soldier before he ascended to kingship, Banquo. One of the prophecies mentions Banquo's predecessors being Kings, as in his son, grandson, great-grandson etc. So Macbeth orders for him and his son to be killed so that he may never produce a child that threatens Macbeth's throne. However Banquo's son survives, and Banquo does not.