Imam Sadiq (peace be upon him): A person said to Imam Sadiq (peace be upon him): "O son of the Messenger of Allah (peace be upon him and his family)! ... Explain to me about the transfer of the Imamate from the progeny of [Imam] Hasan (peace be upon him) to the progeny of [Imam] Husayn (peace be upon him) — why and for what reason did this come about?" The Imam (peace be upon him) said: "The proof of it is this verse of the Quran: 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 thoroughly] (al-Ahzab/33). This verse was revealed concerning five persons, and it bears witness to their purity from polytheism, from the worship of idols, and from the worship of anything other than Allah. Its origin traces back to the prayer of Ibrahim (peace be upon him) where he said: وَ اجْنُبْنِی وَ بَنِیَّ أَنْ نَعْبُدَ الْأَصْنامَ [And keep me and my sons away from worshipping idols] (Ibrahim/35). And the five persons in whose regard the verse of Tathir (purification) 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 and Husayn (peace be upon them), and they are the ones whom the prayer of Ibrahim (peace be upon him) intended. The chief and master of these five in this verse was the Messenger of Allah (peace be upon him and his family). Fatima (peace be upon her) is a woman who shared with them in purification, and [at the same time] she has no share in the Imamate, and she is the mother of the Imams (peace be upon them). When Allah took the soul of His Prophet (peace be upon him and his family), Ali ibn Abi Talib (peace be upon him) was the most deserving of the people for the Imamate after the Messenger of Allah (peace be upon him and his family), on account of this word of Allah the Mighty and Majestic: وَ السَّابِقُونَ السَّابِقُونَ أُولئِکَ الْمُقَرَّبُونَ [And the foremost ones — the foremost — they are those brought near] (al-Waqi'a/10-11), and on account of the word of the Messenger of Allah (peace be upon him and his family) regarding Hasan and Husayn (peace be upon them), who said: 'These two are the masters of the youth of Paradise, and their father is better than them,' and on account of this word of the Prophet (peace be upon him and his family), who said: 'Hasan and Husayn (peace be upon them) are Imams whether they rise or sit, and their father is better than them.' Consequently Ali (peace be upon him) was more deserving of the Imamate than Hasan and Husayn (peace be upon them), because he was the foremost. When Ali (peace be upon him) departed from this world, Hasan (peace be upon him) was more deserving of the Imamate than Husayn (peace be upon him), by the proof of precedence, as He said: وَ السَّابِقُونَ السَّابِقُونَ [And the foremost ones — the foremost] (al-Waqi'a/10). Hasan (peace be upon him) was foremost over Husayn (peace be upon him) and more deserving of the Imamate. When death approached Hasan (peace be upon him), he could not place the Imamate in his own progeny, since his brother (Husayn (peace be upon him)) was like him in purification, and by this merit and by his precedence [over the progeny of Imam Hasan (peace be upon him)] Husayn (peace be upon him) had merit and superiority over the progeny of Hasan (peace be upon him); for that reason the Imamate came to him. When death approached Husayn (peace be upon him), he could not transfer the Imamate to his brother's progeny instead of his own, on account of this verse which says: وَ أُولُوا الْأَرْحامِ بَعْضُهُمْ أَوْلی بِبَعْضٍ فِی کِتابِ اللهِ [And the possessors of relationships are nearer to each other in the ordinance of Allah], since his own progeny were closer to him in kinship than his brother's progeny and were more deserving of the Imamate. So this verse excluded the children of Hasan (peace be upon him) and ruled in favour of the Imamate of the children of Husayn (peace be upon him), and until the Day of Resurrection this Imamate will continue among them, and praise belongs to Allah, the Lord of the worlds."