The purity of Ahl al-Bayt, the family of Muhammad, is manifested by the Verse of Purification in the Qur'an. The development of Shi'ite theology in the period between the death of Muhammad and the disappearance of the Twelfth Imam extends this concept of purity and originates the concept of ismah. The concept of the immunity from sin (ma'sum) of the imams, the Imamiyyah, perhaps began in the first half of the second century AH. Shia scholars of the fourth and the fifth centuries AH extended the infallibility of Muḥammad and the Twelve Imams until the doctrine came to mean that they could not have committed any sin or inadvertent error either before or after they assumed office.