'A'ishah radhiallahu 'anha and the other Mothers of the Believers are included among the Companions of the Prophet sallallahu 'alayhi wa sallam, so every text that forbids slandering the Companions refers to 'A'ishah radhiallahu 'anha too.
Abu Sa'id al-Khudri (may Allah be pleased with him) said: "The Messenger of Allah sallallahu 'alayhi wa sallam said: 'Do not slander my Companions, for if one of you were to spend an amount of gold equivalent to the size of Mount 'Uhud, you would not even come halfway up to their level.' "{qluetip title=[1]}Reported by al-Bukhari, al-Fath, no. 3379){/qluetip}
Moreover, the scholars of Sunni Islam are all agreed that whoever condemns 'A'ishah radhiallahu 'anha for that of which Allah has stated she is innocent is a kafir, because he has rejected Allah's statement of her innocence in Surat an-Nur.
Imam Ibn Hazm quoted a report with an isnad going back to Hisham ibn 'Ammar, who said: "I heard Malik ibn Anas say: 'Whoever curses Abu Bakr should be whipped, and whoever curses 'A'ishah radhiallahu 'anha should be killed.' He was asked, 'Why do you say that concerning (the one who curses) 'A'ishah?' He said, 'Because Allah says concerning 'A'ishah, may Allah be pleased with her (interpretation of the meaning):
'Allah forbids you from it [slander] and warns you not to repeat the like of it forever, if you are believers.' [Al-Qur'an 24:17]"
Malik said: "Whoever accuses her goes against the Qur'an, and whoever goes against the Qur'an should be killed."
Ibn Hazm said: "This comment of Malik's is correct, and it is complete apostasy to reject Allah's Words that clearly state her innocence."
Abu Bakr ibn al-'Arabi said: "Because the people who slandered 'A'ishah radhiallahu 'anha accused a pure and innocent person of immorality, Allah exonerated her. So everyone who accuses her of that of which Allah has stated she is innocent is rejecting what Allah says, and everyone who rejects what Allah says is a kafir. This is the opinion of Malik, and the matter is very clear to those who have insight."
Al-Qadhi Abu Ya'la said: "Whoever slanders 'A'ishah radhiallahu 'anha by accusing her of that of which Allah stated her innocence is a kafir, without doubt. More than one Imam stated this ijma' (consensus) and gave this ruling."
Ibn Abi Musa said: "Whoever accuses 'A'ishah, may Allah be pleased with her, of that of which Allah stated she was innocent has left the religion (is no longer a Muslim) and has no right to marry a Muslim woman."
Ibn Qudamah said: "It is a part of the Sunnah to say 'may Allah be pleased with her' after mentioning the wives of the Prophet sallallahu 'alayhi wa sallam, Mothers of the Believers who are pure and innocent of any evil. The best of them are Khadijah bint Khuwaylid and 'A'ishah radhiallahu 'anha, as-Siddiqah bint as-Siddiq, whose innocence was stated by Allah; (they are) the wives of the Prophet sallallahu 'alayhi wa sallam in this world and the next. Whoever accuses her of that of which Allah has stated her innocence has rejected the words of Allah All-Mighty."
Imam an-Nawawi, may Allah have mercy on him, said: "'A'ishah's innocence of that of which she was accused is stated definitively in the Qur'an. If anyone doubts that (may Allah protect us from such a thing), he becomes a kafir and an apostate, by the consensus of the Muslims."
Ibn al-Qayyim, may Allah have mercy on him, said: "The Ummah is agreed that whoever slanders her is a kafir."
Al-Hafidh Ibn Kathir said, in his tafsir: "The scholars, may Allah have mercy on them, all agreed that whoever accuses or slanders her after the revelation of this ayah is a kafir, because he has rejected the Qur'an."
Badr ad-Din az-Zirkashi said: "Whoever slanders her is a kafir, because the Qur'an clearly states her innocence."
The scholars based their ruling on the one who slanders 'A'ishah radhiallahu 'anha on the following evidence:
1. The evidence that is derived from the verses in Surat an-Nur that clearly state her innocence. So whoever accuses her after Allah has declared her innocent is rejecting the words of Allah, which is kufr beyond any shadow of a doubt.
(2) Slandering the family of the Prophet sallallahu 'alayhi wa sallam hurts and offends the Prophet himself, and there is no doubt that whatever hurts and offends the Prophet (peace be upon him) is kufr, by consensus (ijma'). Evidence that the slander of his wife hurt and offended the Prophet sallallahu 'alayhi wa sallam is seen in the hadith of the slander (al-ifk) reported by al-Bukhari and Muslim, in which 'A'ishah radhiallahu 'anha says: " ... The Messenger of Allah sallallahu 'alayhi wa sallam stood up on that day and asked who would go and deal with 'Abdullah ibn Ubayy. He was on the minbar, and said: 'O Muslims, who will deal with a man who I have heard is speaking in an offensive manner about my family? By Allah, I know nothing but good about my family.' ... " What the Prophet sallallahu 'alayhi wa sallam meant was: who will be kind to me, and excuse me if I go and deal with him myself, and I give him what he deserves because I have heard that he is speaking in an offensive manner about my family. This proves that the Prophet sallallahu 'alayhi wa sallam was so deeply offended and hurt that he asked people whether they could deal with this person fairly.
Imam al-Qurtubi said, in his tafsir of the ayah "Allah forbids you from it [slander] and warns you not to repeat the like of it forever, if you are believers." [Al-Qur'an 24:17]:
"This is concerning 'A'ishah radhiallahu 'anha ... because of the hurt and offence that the Messenger of Allah sallallahu 'alayhi wa sallam felt with regard to his honour and his family. This is kufr on the part of the one who does it."
(3) Slandering 'A'ishah radhiallahu 'anha implies insulting the Prophet sallallahu 'alayhi wa sallam, because Allah, may He be glorified, says (interpretation of the meaning):
"Bad statements are for bad people (or bad women for bad men) and bad people for bad statements (or bad men for bad women) ... " [Al-Qur'an 24:26]
Al-Hafidh Ibn Kathir, may Allah have mercy on him, said: "I.e., Allah would not have made 'A'ishah radhiallahu 'anha the wife of the Messenger of Allah sallallahu 'alayhi wa sallam if she had not been good, because he is better than any good person. If she had been bad, she would not have been fit to marry him from a shar'i point of view, and Allah would never even have decreed it."
Finally, let us remember that the most beloved of all people to him sallallahu 'alayhi wa sallam was 'A'ishah radhiallahu 'anha, as-Siddiqah bint as-Siddiq, as is proven in the report of 'Amr ibn al-'As, who said: "The Messenger of Allah sallallahu 'alayhi wa sallam put me in charge of an army during the ghazwah (campaign) of as-Salasil. I came to him and asked him, 'O Messenger of Allah, who among the people is most beloved to you?' He said, ''A'ishah.' I asked, 'Who among men?' He said, 'Her father.' I asked, 'Then who?' He said, ''Umar,' then he mentioned a number of others."
So whoever feels hatred towards the beloved of the Messenger of Allah sallallahu 'alayhi wa sallam will deserved to be despised by him on the Day of Resurrection. And Allah knows best.{qluetip title=[2]}See 'Aqidat Ahlus-Sunnah wa'l-Jama'ah fi'l-Sahabah al-Kiram by Nasir ash-Shaykh, 2/781, and I'tiqad Ahlus-Sunnah fi'l-Sahabah by Muhammad al-Wahaybi, p. 58.{/qluetip}