.Ayat al-Tat'hir:
"Verily Allah intends to keep off from you every kind of uncleanness O' People of the House (Ahlul-Bayt) , and purify you with a perfect purification". (Quran, 33:33)
Narrated Aisha: One day the Prophet (s) came out afternoon wearing a black cloak (upper garment or gown; long coat), then al-Hasan Ibn Ali came and the Prophet accommodated him under the cloak, then al-Husain came and entered the cloak, then Fatimah came and the Prophet entered her under the cloak, then Ali came and the Prophet entered him to the cloak as well. Then the Prophet recited: "Verily Allah intends to keep off from you every kind of uncleanness O' People of the House (Ahlul-Bayt) , and purify you a perfect purification (the last sentence of Verse 33:33)."
Reference:
Sahih Muslim, Chapter of virtues of companions, section of the virtues of the Ahlul-Bayt of the Prophet (s), 1980 Edition Pub. in Saudi Arabia, Arabic version, v4, p1883, Tradition #61.
One can see that the author of Sahih Muslim confirms that:
Imam Ali, Fatimah, al-Hasan, and al-Husain are the Ahlul-Bayt,
The purification sentence in Quran (the last sentence of Verse 33:33) was revealed for the virtue of the above-mentioned individuals.
He (Muslim) did not put any other tradition in this section (section of the virtues of Ahlul-Bayt). If the author of Sahih Muslim believed that the wives of the Prophet were included in Ahlul-Bayt, he would have quoted some traditions about them in this section.
It is interesting to see that Aisha, the wife of the Prophet (s) is the narrator of the above tradition, and she herself is testifying that Ahlul- Bayt are the above-mentioned individuals (i.e., Imam Ali, Fatimah, al-Hasan, and al-Husain)
Sahih al-Tirmidhi, v5, pp 351,663
Another version of the "Tradition of Cloak" is written in Sahih al-Tirmidhi, which is narrated in the authority of Umar Ibn Abi Salama, the son of Umm Salama (another wife of Prophet), which is as follows:
The verse "Verily Allah intends to ... (33:33)" was revealed to the Prophet (s) in the house of Umm Salama. Upon that, the Prophet gathered Fatimah, al-Hasan, and al-Husain, and covered them with a cloak, and he also covered Ali who was behind him. Then the Prophet said: "O' Allah! These are the Members of my House (Ahlul-Bayt) . Keep them away from every impurity and purify them with a perfect purification." Umm Salama (the wife of Prophet) asked: "Am I also included among them O Apostle of Allah?" the Prophet replied: "You remain in your position and you are toward a good ending."
al-Mustadrak, by al-Hakim, v2, p416
Musnad Ahmad Ibn Hanbal, v6, pp 323,292,298; v1, pp 330-331; v3, p252; v4, p107 from Abu Sa'id al-Khudri
Fadha'il al-Sahaba, by Ahmad Ibn Hanbal, v2, p578, Tradition #978
al-Mustadrak, by al-Hakim, v2, p416 (two traditions) from Ibn Abi Salama, v3, pp 146-148 (five traditions), pp 158,172
al-Khasa'is, by an-Nisa'i, pp 4,8
al-Sunan, by al-Bayhaqi, narrated from Aisha and Umm Salama
Tafsir al-Kabir, by al-Bukhari (the author of Sahih), v1, part 2, p69
Tafsir al-Kabir, by Fakhr al-Razi, v2, p700 (Istanbul), from Aisha
Tafsir al-Durr al-Manthoor, by al-Suyuti, v5, pp 198,605 from Aisha and Umm Salama
Tafsir Ibn Jarir al-Tabari, v22, pp 5-8 (from Aisha and Abu Sa'id al-Khudri), pp 6,8 (from Ibn Abi Salama) (10 traditions)
Tafsir al-Qurtubi, under the commentary of verse 33:33 from Umm Salama
Tafsir Ibn Kathir, v3, p485 (Complete version) from Aisha and Umar Ibn Abi Salama
Usdul Ghabah, by Ibn al-Athir, v2, p12; v4, p79 narrated from Ibn Abi Salama
Sawa'iq al-Muhriqah, by Ibn Hajar al-Haythami, Ch. 11, sec. 1, p221 from Umm Salama
Tarikh, by al-Khateeb Baghdadi, v10, narrated from Ibn Abi Salama
Tafsir al-Kashshaf, by al-Zamakhshari, v1, p193 narrated from Aisha
Mushkil al-Athar, by al-Tahawi, v1, pp 332-336 (seven traditions)
Dhakha'ir al-Uqba, by Muhibb al-Tabari, pp21-26, from Abu Sa'id Khudri
Majma' al-Zawa'id, by al-Haythami, v9, p166 (by several transmitters) .... and more ....
The fact that the gender in later part of Verse 33:33 is switched from feminine to masculine, has led the majority of Sunni commentators to believe that the last part was revealed for Imam Ali, Fatimah, al-Hasan and al-Husain, as Ibn Hajar al-Haythami indicated:
Based on the opinion of the majority of commentators, the saying of Allah :"Verily Allah intends to ... (the last sentence of the verse 33:33)" was revealed for Ali, Fatimah, al-Hasan, and al-Husain, because of the usage of masculine gender in the word "Ankum" and after that.
Reference: al-Sawa'iq al-Muhriqah, by Ibn Hajar, Ch. 11, section 1, p220.