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Objection to joining the daughter of the Messenger of Allah and the daughter of the enemy of Allah together as wives of one man.
As the Prophet (blessings and peace of Allah be upon him) said: “By Allah, the daughter of the Messenger of Allah and the daughter of the enemy of Allah will not be joined together as wives of one man.”
An-Nawawi said: It was said that what is meant here is not the prohibition on them being joined together; rather what is meant is: I know by the grace of Allah that they will not be joined together.
It may also be understood as meaning that it was haraam to join them together… So one of the things that are haraam with regard to marriage is being married to both a daughter of the Prophet of Allah and a daughter of the enemy of Allah at the same time.
End quote from Sharh Saheeh Muslim, 8/199
Ibn al-Qayyim said: By disallowing ‘Ali to join together Faatimah (may Allah be pleased with her) and the daughter of Abu Jahl as co-wives there is great wisdom: because a woman will follow her husband and be at the same level as him in Paradise, but she may deserve to be of a high level by virtue of her own merits in addition to those of her husband. This is applicable in the case of Faatimah and ‘Ali (may Allah be pleased with them both).
But Allah, may He be glorified and exalted, would not allow the daughter of Abu Jahl to be of the same status as Faatimah (may Allah be pleased with her), whether on her own merits or those of her husband, when there was such a great difference between them. For her to become a co-wife of the leader of the women of the worlds would not be something appropriate, either in terms of sharee‘ah or in terms of the divine decree.
The Prophet (blessings and peace of Allah be upon him) indicated that when he said: “By Allah, the daughter of the Messenger of Allah and the daughter of the enemy of Allah will not be joined together in one place.” This may refer to the Hereafter too.
End quote from Zaad al-Ma‘aad, 5/119
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Out of respect for Faatimah and so as to highlight her great status.
Ibn Hibbaan said: If ‘Ali has done this deed, it would have been permissible, but the Prophet (blessings and peace of Allah be upon him) disliked it out of respect for Faatimah, not because this deed was haraam.
End quote from Saheeh Ibn Hibbaan, 15/407
Al-Haafiz Ibn Hajar said: The context indicates that it was permissible for ‘Ali, but the Prophet (blessings and peace of Allah be upon him) disallowed it out of care for Faatimah’s feelings, and ‘Ali accepted it out of obedience to the instructions of the Prophet (blessings and peace of Allah be upon him). Thus it seems to me that it is not far-fetched to suggest that one of the unique characteristics of the Prophet (blessings and peace of Allah be upon him) was that his sons-in-law were not to marry other wives when they were married to his daughters. Or it may be that this is something that applied only to Faatimah (may Allah be pleased with her).
End quote from Fath al-Baari, 9/329
In conclusion:
These reasons, as a whole or individually, explain why the Prophet (blessings and peace of Allah be upon him) did not let ‘Ali go ahead with this marriage.
This story does not give the slightest support to those who try to use it to restrict plural marriage. The Prophet (blessings and peace of Allah be upon him) dealt with this when he said, in the same story, “and I am not making any permissible thing forbidden, or any forbidden thing permitted.”
Narrated by al-Bukhaari, 3110; Muslim, 2449
And Allah knows best.