Imam Rida (peace be upon him): Rayyan ibn Salt narrates: Imam Rida (peace be upon him) attended al-Ma'mun's gathering in Marw, and a group of the scholars of Iraq and Khurasan were present in that gathering... The scholars present said: "Now tell us: has Allah, the Blessed and Exalted, explained the matter of 'istifa' (chosen selection) in the Quran?" He (peace be upon him) said: "Allah, the Blessed and Exalted, has explained this matter in the outward of the Quran, apart from the inward, in twelve places," and he began to mention those twelve places until he reached the twelfth. And he said: "As for the twelfth instance, it is the verse: وَ أْمُرْ أَهْلَکَ بِالصَّلَاةِ وَ اصْطَبِرْ عَلَیْهَا [And enjoin prayer on your followers, and steadily adhere to it], and Allah has singled us out with this distinction. For He commanded us, along with the ummah, to establish prayer, and then in addition He specifically obligated us to perform it. For this reason, the Messenger of Allah (peace be upon him and his family), for nine months after the revelation of this verse, would come to the door of the house of Ali and Fatima (peace be upon them) after every prayer time and would say: 'It is the time of prayer, may Allah have mercy on you.' Allah has not favored the offspring of any prophet with such an honor as He has granted us and singled us out with." Then al-Ma'mun and the other scholars said: "May Allah reward you, the Ahl al-Bayt of your Prophet (peace be upon him and his family), with good on behalf of this ummah."