Imam Sadiq (peace be upon him): A questioner asked him... 'Inform me about the imamate passing from the descendants of Imam Hasan (peace be upon him) and being placed among the descendants of Imam Husayn (peace be upon him) — how is that, and what is the proof of it?' He said: 'The word of Allah, the Blessed and Exalted: إِنَّما یُرِیدُ اللهُ لِیُذْهِبَ عَنْکُمُ الرِّجْسَ أَهْلَ الْبَیْتِ وَ یُطَهِّرَکُمْ تَطْهِیراً [Allah only wishes to remove impurity and sin from you, O People of the House, and to purify you completely] (Al-Ahzab/33). This verse was revealed concerning five persons, and it bore witness to their purity from shirk (polytheism), from the worship of idols, and from the worship of anything other than Allah. Its origin is the supplication of Ibrahim (peace be upon him) where he says: وَ اجْنُبْنِی وَ بَنِیَّ أَنْ نَعْبُدَ الْأَصْنامَ [And keep me and my sons away from worshipping idols]. And the five persons concerning whom the verse of purification (ayat al-tathir) was revealed are: the Messenger of Allah (peace be upon him and his family), Ali (peace be upon him), Fatima (peace be upon her), Hasan (peace be upon him) and Husayn (peace be upon him), and they are the ones intended by the supplication of Ibrahim (peace be upon him). Their master in that [verse] was the Messenger of Allah (peace be upon him and his family), and Fatima (peace be upon her) was a woman who shared with them in the purification, and she has no part in the imamate, but she is the mother of the Imams (peace be upon them). When Allah took the soul of His Prophet, Ali ibn Abi Talib (peace be upon him) was, after the Messenger of Allah (peace be upon him and his family), the most deserving of people for the imamate — because of the word of Allah the Mighty and Majestic: وَ السَّابِقُونَ السَّابِقُونَ أُولئِکَ الْمُقَرَّبُونَ [And the foremost, the foremost — these are the ones brought near], and because of the word of the Messenger of Allah (peace be upon him and his family) concerning Hasan (peace be upon him) and Husayn (peace be upon him): the two of them are the masters of the youth of the people of Paradise, and their father is better than them; and because of the saying of the Prophet (peace be upon him and his family): Hasan (peace be upon him) and Husayn (peace be upon him), whether they rise up or sit still, are rightful imams, and their father is better than them. Therefore Ali (peace be upon him) was more deserving of the imamate than Hasan (peace be upon him) and Husayn (peace be upon him), because he was foremost [in faith and good deeds]. And when he departed this world, Hasan (peace be upon him) was more deserving of the imamate than Husayn (peace be upon him) by virtue of being foremost, and that is the word of Allah: وَ السَّابِقُونَ السَّابِقُونَ [And the foremost, the foremost]. Hasan (peace be upon him) was more foremost than Husayn (peace be upon him) and more deserving of the imamate. And when the death of Hasan (peace be upon him) drew near, it was not permissible for him to place it among his own children, while his brother was his equal in purification and possessed, by virtue of that [purification] and by virtue of being foremost, superiority over the children of Hasan (peace be upon him).'