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]
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / Q2861
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
(a) Sayyiduna Abu Hurayrah (may Allah be pleased with him) says that the Messenger of Allah (peace and blessings be upon him) said: "O Allah, make the provision of the family of Muhammad just enough to live on."
(b) The Prophet (peace and blessings be upon him) said: "The family of Muhammad only eat from this wealth; they have nothing more than this food." By this he (peace and blessings be upon him) meant those under his maintenance.
(c) It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that the family of Muhammad (peace and blessings be upon him) never ate their fill of food for three consecutive days until he (peace and blessings be upon him) departed from this world.
(d) It is narrated from Sayyidah Aishah (may Allah be pleased with her) that from the time the family of Muhammad (peace and blessings be upon him) came to Madinah, they never ate their fill of wheat bread for three days until he (peace and blessings be upon him) departed from this transient world.
(e) It is narrated from Sayyidah Aishah (may Allah be pleased with her): We, the family of Muhammad (peace and blessings be upon him), would spend a whole month without a fire being lit in our houses; our food was dates and water.
(f) Abu Abdullah al-Halimi (may Allah have mercy on him) indicated that the term 'Ahl al-Bayt' (household) applies to the wives in its true sense, while applying the term 'Al' (family) to them is by way of likening to lineage, and especially to the wives of the Prophet (peace and blessings be upon him), because their bond with the Prophet (peace and blessings be upon him) is never severed, and they (the pure wives) are unlawful for anyone else both during his (peace and blessings be upon him) lifetime and after his (peace and blessings be upon him) death; for this reason their tie with the Prophet (peace and blessings be upon him) stands established (like lineage).
Imam al-Bayhaqi (may Allah have mercy on him) says: The Prophet's (peace and blessings be upon him) explicit command to send blessings upon his wives makes any other evidence unnecessary.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2862
Sayyiduna Abu Humayd al-Sa'idi (may Allah be pleased with him) says: We said: O Messenger of Allah, how should we send blessings upon you? The Messenger of Allah (peace and blessings be upon him) said: "Say: 'Allahumma salli 'ala Muhammadin wa azwajihi wa dhurriyyatih, kama sallayta 'ala ali Ibrahim, wa barik 'ala Muhammadin wa azwajihi wa dhurriyyatih, kama barakta 'ala ali Ibrahim, innaka Hamidun Majid' (O Allah, send Your mercy upon Muhammad (peace and blessings be upon him), upon his pure wives and upon his offspring, just as You sent mercy upon the family of Ibrahim (peace be upon him); and send down blessing upon Muhammad (peace and blessings be upon him), upon his wives and upon his offspring, just as You sent down blessing upon the family of Ibrahim (peace be upon him). Indeed, You are Praiseworthy, Glorious)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2863
It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that he (peace and blessings be upon him) said: "Whoever would be pleased to have his deeds weighed out in full measure and be recompensed accordingly, then when he sends blessings upon us, the people of the household, let him say: 'Allahumma salli 'ala Muhammadin an-nabiyyi wa azwajihi ummahatil-mu'minina wa dhurriyyatihi wa ahli baytih, kama sallayta 'ala Ibrahim, innaka Hamidun Majid' (O Allah, send Your mercy upon Muhammad (peace and blessings be upon him), who is the Prophet, upon his wives, who are the Mothers of the Believers, and upon his offspring and his household, just as You sent mercy upon Ibrahim (peace be upon him). Indeed, You are Praiseworthy, Glorious)."
It is as though he (peace and blessings be upon him) mentioned his wives and his offspring separately by way of emphasis, then used a general term so as to include in it his household besides the wives and offspring; and may there be mercy upon all of them.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2866