This is because Allah Almighty addressed them (the pure wives) with His words: "O wives of the Prophet, you are not like any other of the women. If you fear Allah, then do not be soft in speech." [Surah al-Ahzab: 32]
Then He continued the discourse until He said: "Allah only wishes to remove from you every kind of impurity, O people of the household, and to purify you thoroughly." [Surah al-Ahzab: 33]
And Allah Almighty used the word "ankum" (from you) in the masculine form because the intent was to include others (of the household) along with them (the pure wives) in this ruling. Then, attributing the houses to them, He said: "And remember what is recited in your houses of the verses of Allah and wisdom." [Surah al-Ahzab: 34]
Sayyidah Umm Salamah (may Allah be pleased with her) says that this verse was revealed in my house, namely: "Allah only wishes to remove impurity from you, O people of the household, and to purify you thoroughly" [Surah al-Ahzab: 33].
Sayyidah Umm Salamah (may Allah be pleased with her) says: The Messenger of Allah (peace and blessings be upon him) called Fatimah, Ali, Hasan and Husayn (may Allah be pleased with them) and said: "These are my household."
(b) In the hadith of al-Qadi and al-Sulami it is: "These are my family." The Mother of the Believers Umm Salamah (may Allah be pleased with her) says: I said: "O Messenger of Allah, am I not among the household?" He (peace and blessings be upon him) said: "Of course, you are included too, if Allah wills."
(c) Imam al-Bayhaqi (may Allah have mercy on him) says that this hadith has supporting reports and also contrary ones, the like of which are not established. In the Book of Allah there is the clarification of what we intended; and from the Prophet's (peace and blessings be upon him) unqualified mention of the 'Al' (family) while meaning his wives by it, it is known that they too are included in the 'Al'.
Abu Abdullah (al-Hakim) said: This hadith is sahih (authentic); its narrators are thiqat (trustworthy).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2861