The authentic tradition reported by Shaykh al-Kulayni, may Allah bless him… Abu Abdillah al-Sadiq (a.s.), who said the following in the course of mentioning the Imams… ‘Thus Allah continued to choose them (the Imams) for His creatures from the progeny of al-Husain (a.s.) from the offspring of every Imam. Whenever an  Imam would pass away from among them, He (Allah) would appoint for His creatures an Imam, and a guiding authority from his (i.e. the previous Imam’s) offspring…’