Imam Sadiq (peace be upon him): Allah the Mighty and Majestic transferred the imamate to the descendants of Husayn (peace be upon him), just as He transferred prophethood from the descendants of Ishaq to the descendants of Isma'il (peace be upon him). The ummah is in agreement concerning the descendants of Husayn (peace be upon him) and bears witness to them and to the fact that the imamate flows among them, and they have not reached such a consensus of testimony for anyone besides them. So if a speaker says: "What is the proof that Allah the Mighty and Majestic transferred the imamate from the descendants of Hasan (peace be upon him) to the descendants of Husayn (peace be upon him)?" we say to him: "The Book of Allah has transferred it." And if he says: "How is that? The imamate is only through precedence [in faith] and purity from the destructive sins that make the Fire binding, and then through manifest knowledge," he is told: "[The transfer of] the imamate encompasses everything the ummah needs, namely: what is lawful, what is unlawful, and knowledge of the Book of Allah — [including] its particular and general, its outward and inward, its firm and ambiguous, its abrogating and abrogated, the subtleties of its knowledge and the rare aspects of its interpretation." The questioner asked: "What is the proof that the imam cannot be other than one who knows these things that you have mentioned?" He said: "The saying of Allah the Mighty and Majestic concerning those whom He permitted to judge and made worthy of it [where He said:] إِنَّا أَنْزَلْنَا التَّوْراةَ فِیها هُدیً وَ نُورٌ یَحْکُمُ بِهَا النَّبِیُّونَ الَّذِینَ أَسْلَمُوا لِلَّذِینَ هادُوا وَ الرَّبَّانِیُّونَ وَ الْأَحْبارُ [Surely We revealed the Tawrat in which was guidance and light; with it the prophets who submitted themselves (to Allah) judged (matters) for those who were Jews, and the masters of Divine knowledge and the doctors]. So the 'Rabbaniyyun' are the imams, who — being below the prophets — nurture people by their knowledge, and the 'Ahbar' are below them, and they are callers on behalf of the imams. Then Allah the Mighty and Majestic informed and said: بِمَا اسْتُحْفِظُوا مِنْ کِتابِ اللهِ وَ کانُوا عَلَیْهِ شُهَداءَ [because they were required to guard (part) of the Book of Allah, and they were witnesses thereof], and He did not say: 'because of their ignorance.' Then He said: 'Are those who know equal to those who do not know? Only those endowed with understanding take heed.' (Zumar/9) And He said: 'Rather, these are clear signs in the breasts of those who have been given knowledge.' (Ankabut/49) And He said: 'And none understand them except the knowledgeable.' (Ankabut/43). Then He said: 'Indeed, only those of His servants who have knowledge fear Allah.' (Fatir/28) And He said: 'Is then He Who guides to the truth more worthy to be followed, or he who himself does not go aright unless he is guided? What then is the matter with you? How do you judge?!' (Yunus/35). So these verses are proof that the imams cannot be other than knowledgeable ones, so that people are in need of them while they are not in need of any of the people in anything of the lawful and unlawful."