Ruling on slandering the daughter of the Prophet (blessings and peace of Allah be upon him)
Question

What is our belief about defending the honour of the Prophet (blessings and peace of Allah be upon him)? What is the ruling on one who slanders his daughter and not his wives, based on evidence from the Qur’an and Sunnah, and the words of the scholars?
Praise be to Allah.Firstly:
Respecting the Prophet (blessings and peace of Allah be upon him) is obligatory for every Muslim
It is obligatory for the Muslims to respect the Prophet (blessings and peace of Allah be upon him), venerate him and declare him to be above all shortcomings. Allah, may He be exalted, says (interpretation of the meaning):
{Indeed, We have sent you as a witness and a bringer of good tidings and a warner,
That you [people] may believe in Allah and His Messenger and honor him and respect the Prophet} [al-Fath 48:8-9].
Al-Qadi ‘Iyad (may Allah have mercy on him) said:
We have quoted above from the Qur’an and Sunnah, and the consensus of the ummah, the rights that the Prophet (blessings and peace of Allah be upon him) has over us, and what he is entitled to of honour, respect, veneration and esteem. Based on that, Allah, may He be exalted, in His Book, has forbidden speaking ill of him, and the ummah it is unanimously agreed that any Muslim who disparages him or slanders him is to be executed… "(Ash-Shifa  p. 760).
Secondly:
Deliberately speaking ill of the Prophet (blessings and peace of Allah be upon him) constitutes disbelief (kufr)
The religious texts indicate that deliberately speaking ill of the Prophet (blessings and peace of Allah be upon him) constitutes disbelief (kufr). That includes the verses in which Allah, may He be exalted, says of the hypocrites (interpretation of the meaning):
{And among them are those who offend the Prophet and say, “He is an ear.” Say, "”[It is] an ear of goodness for you that believes in Allah and believes the believers and [is] a mercy to those who believe among you.” And those who offend the Messenger of Allah - for them is a painful punishment.
They swear by Allah to you [Muslims] to satisfy you. But Allah and His Messenger are more worthy for them to satisfy, if they should be believers.
Do they not know that whoever opposes Allah and His Messenger - that for him is the fire of Hell, wherein he will abide eternally? That is the great disgrace} [at-Tawbah 9:61-63].
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said:
Thus it is known that speaking ill of the Messenger of Allah is opposing Allah and His Messenger, because offending is regarded as opposing. Thus the connection is clear, otherwise the beginning of the verse would not be in harmony, if we were to suggest that offending is not regarded as opposing.
This indicates that offending and opposing the Prophet (blessings and peace of Allah be upon him) constitutes disbelief (kufr), because here Allah tells us that the one who does that will be thrown into the fire of Hell, to abide therein forever, and He did not say that that will be his punishment [because one who is punished in the Fire may be brought forth from it]. There is a difference between the two, because opposing is taking someone as an enemy, and that is disbelief and waging war, so it is more grievous than mere disbelief. Thus the one who speaks ill of the Messenger of Allah (blessings and peace of Allah be upon him) and offends and insults him is an enemy to Allah and His Messenger, and is waging war against Allah and His Messenger… "(As-Sarim al-Maslul  2/58).
There is scholarly consensus on this point.
Al-Qadi ‘Iyad (may Allah have mercy on him) said:
Whoever accuses our Prophet (blessings and peace of Allah be upon him) of deliberately lying in what he conveyed and spoke of, or doubts his truthfulness, or reviles him, or says that he did not convey the message, or shows disrespect towards him or any of the Prophets, or says bad things about them, or speaks ill of them, or killed or fought a prophet is a disbeliever (kafir) according to scholarly consensus."(Ash-Shifa  2/284).
Speaking ill of the daughter is a great offence to the father.
As the Messenger of Allah (blessings and peace of Allah be upon him) said regarding his daughter Fatimah (may Allah be pleased with her): “She is part of me; what offends her offends me, and what hurts her hurts me.” Narrated by al-Bukhari.
Al-Hafiz Ibn Hajar (may Allah have mercy on him) said:
This hadith indicates that it is haram to offend anyone who the Prophet (blessings and peace of Allah be upon him) will be offended if that person is offending, because offending the Prophet (blessings and peace of Allah be upon him) is haram according to consensus, whether it is to a lesser or greater extent, because he confirmed that what offended Fatimah offended him. Therefore anyone who said or did anything that led to Fatimah being offended, that would also offend the Prophet (blessings and peace of Allah be upon him), based on this sahih report."(Fath al-Bari  9/329).
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said:
If someone does something that could offend the Prophet without the doer knowing that it could offend him and the doer did not mean to offend him, he should be told to stop it and his action is a sin, such as raising his voice above the Prophet’s. As for deliberately offending him or doing something that the doer knows would offend him, but he went ahead and did it with full awareness, this is what constitutes disbelief (kufr) and the erasure of good deeds. And Allah, may He be glorified, knows best."(As-Sarim al-Maslul  2/120).
Especially if the offence takes the form of slander, because people naturally realise that slander of the daughter would undermine the father’s status among people, and he would be shamed by that.
Undoubtedly the one who slanders the daughter of the Messenger of Allah (blessings and peace of Allah be upon him) causes great offence to her father (blessings and peace of Allah be upon him) and would undermine his status among people, so deliberately doing that constitutes disbelief (kufr).
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said:
Abu Bakr ibn Ziyad an-Naysaburi said: I heard al-Qasim ibn Muhammad say to Isma‘il ibn Ishaq: Two men were brought to al-Ma’mun in ar-Raqqah, one of whom had slandered Fatimah and the other had slandered ‘A’ishah. He ordered that the one who had slandered Fatimah be executed, and he released the other one. Isma‘il said: Both of them should be executed, because the one who slanders ‘A’ishah has rejected the Qur’an. This is the ruling of the jurists and scholars among Ahl al-Bayt and others."(As-Sarim al-Maslul  3/1051.
Imam Abu’l-Qasim as-Suhayli (may Allah have mercy on him) said: … The Messenger of Allah (blessings and peace of Allah be upon him) said: “Fatimah is part of me,” so send blessings upon him and upon Fatimah.
This hadith indicates that the one who reviles her has committed an act of disbelief (kufr), and the one who sends blessings upon her has sent blessings upon her father (blessings and peace of Allah be upon him)."(Ar-Rawd al-Unuf  6/328).
And Allah knows best.