The demise of Syedah Fatimah Al Zahraa is attributable to the treatment of her at the hands of the Companions of the Prophet Muhammad (pbuhahf) after he passed away from this world. Historians generally state her age as less than 30 at the time.
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To summarise:

• When Imam Ali (as) refused to give allegiance to the Companions that they had begun demanding after the demise of the Prophet (pbuhahf), his supporters congregated inside the home of Syedah Fatimah (as)
• She was in support of Imam Ali (as), since she was defending his rights to the caliphate, as per the Prophet’s written Will
• This infuriated the Companions who decided to attack her home
• With the permission of Abu Bakr, Umar bin Al Khattab led the group that attacked her home during the night
• When they attacked her home, they tried to burn it down and they managed to break into the front door
• She was pinned between the main door and the wall and one nail in the door stabbed her in the chest
  She was also beaten and whipped: some historians say it was Qunfuz, some say it was Umar
• Her health deteriorated after this attack and though the Companions came to her to seek forgiveness, she never forgave them
• Her final wishes were to be buried at night so that the enemies of Allah who had wreaked havoc in Islam, could not pray over her
• Imam Ali complied with her wishes and even he stated that he had not realised the extent of her injuries
• Syed Ahmed Al Hasan (as) has clarified that Imam Ali buried her in Jannat Al Baqee, Madinah: her current position is adjacent to the grave of Imam Hasan (as) “as if she is cradling him”

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Below are excerpts from the books that give commentary on the above by Syed Ahmed Al Hasan (as)…

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Wilderness of the Path to Allah

“The matter crossed the line with an attempt by Umar b. Al Khattab and a group of hypocrites to burn the house of Fatimah Al Zahra’ (as), though she is the only daughter of Allah’s Messenger (pbuhahf) from his progeny, and she, Al Hasan, Al Hussain, Ali (pbut) are those about whom Allah has obligated their love in the Qur’an. Allah Most High said, (Say, I do not ask any reward from you for it, other than love of the close ones)

When this attempt did not benefit them in forcing the Imam (pbuh) out to forcefully give allegiance to Abu Bakr, they broke into the home, attacking Al Zahraa (pbuh) and they broke her rib and caused her to miscarry her unborn child and the nail was buried in her chest, even though Allah’s Messenger (pbuhahf) had said about her: “The mother of her father…a part of me…Allah is pleased with her pleasure and angered by her anger…leader of the women of all the worlds, from the first and the last. So the Muslims would not be able to escape the wilderness, and it would not escape them, after they travelled the same path as the Children of Israel – in their footsteps – Allah’s Messenger (pbuhahf) said “By him in whose hand my life is, you will follow the traditions of those before you – exactly in their footsteps – until you cannot escape their path and it does not escape you: tradition of the Children of Israel” (The Wilderness or the Path to Allah by Imam Ahmed Al Hasan (as)

بل تجاوز الأمر إلى محاولة عمر بن الخطاب وجماعة من المنافقين إحراق بيت فاطمة الزهراء (ع) وهي بنت رسول الله (ص) الوحيدة من صلبه، وهي والحسن والحسين وعلي (ع) من فرض الله مودتهم في القرآن، قال تعالى:
(قُلْ لا أَسْأَلُكُمْ عَلَيْهِ أَجْراً إِلَّا الْمَوَدَّةَ فِي الْقُرْبَى)
ولما لم تنفع هذه المحاولة لإخراج الإمـام (ع) لبيعة أبي بكر كرها اقتحموا الدار على الزهراء (ع) وكسروا ضلعها واسقطوا جنينها ونبت المسمار في صدرها، وهي التي قال فيها رسول الله (ص): )أم أبيها وبضعة مني ويرضى الله لرضاها ويغضب لغضبها وسيدة نساء العالمين من الأولين والآخرين)
فلم يكن المسلمون ليخطئوا التيه أو ليخطئهم بعد أن ساروا على نفس طريق بني إسرائيل حذو النعل بالنعل، قال رسول الله (ص) : (والذي نفسي بيده لتركبن سنن من كان قبلكم حذو النعل بالنعل والقذة بالقذة حتى لا تخطئون طريقهم ولا يخطئكم سنة بني إسرائيل)
التيه أو الطريق إلى الله

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The Calf

And Ali b. Abi Talib (pbuh), the Executor of Allah’s Messenger (pbuhahf), was pushed aside, and he and Al Zahra’ (pbut) were harmed, and she passed away as the result of the attack on her home at the hands of Umar and a group of hypocrites, to force Imam Ali (pbuh) to pledge allegiance to Abu Bakr, as well as his beating of her with a whip and squeezing her between the wall and the door, until her rib was broken and a nail was buried in her chest and she miscarried her unborn baby. She went to her father having been treated with cruelty, subjugated by her people, though they would often hear the Prophet (pbuh) say a statement whose meaning was “Allah is angered by the anger of Fatimah”

So curse on a people for their breaking of Allah’s sanctity, and for them belittling the best of creation. (Their example is that of the one who lights a fire, then when it enlightens everything around him, Allah takes away their light and leaves them in darkness, they cannot see) (Surah Al Baqarah)

In addition to this the Prophet (pbuhahf) did not leave the Muslims during his life without turning their attention to leadership after him, and towards the executors from his progeny (pbut), the way that Allah, be He glorified, commanded him.

The Calf: vol. 1 (official – partial translation)

The Calf: vol. 1 (unofficial – full translation)

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ونُحي وصي رسول الله (ص) علي بن أبي طالب (ع)، وأوذي هو والزهراء (عليهما السلام)، وماتت (ع) أثر اقتحام عمر وجماعة من المنافقين دارها لإجبار الإمام علي (ع) على مبايعة أبي بكر، وضربها بالسوط وضغطها بين الحائط والباب حتى كسر ضلعها ونبت المسمار في صدرها وأسقط جنينها. ووردت على أبيها مظلومة، مقهورة من قوم كانوا يسمعون النبي (ص) يقول ما معناه
(إنّ الله يغضب لغضب فاطمة)
فتعساً للقوم لما انتهكوا من حرم الله، واستخفوا بخيرة خلقه
(مَثَلُهُمْ كَمَثَلِ الَّذِي اسْتَوْقَدَ نَاراً فَلَمَّا أَضَاءَتْ مَا حَوْلَهُ ذَهَبَ اللَّهُ بِنُورِهِمْ وَتَرَكَهُمْ فِي ظُلُمَاتٍ لا يُبْصِرُونَ)
هذا والنبي (ص)لم يترك المسلمين في حياته دون أن يوجههم إلى القيادة من بعده، وإلى الأوصياء من ولده (ع)، حيث أمره الله سبحانه بذلك)
كتاب العجل ج 1

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With the Pious Servant

Remembering the martyrdom of Al Zahraa…
He (Syed Ahmed Al Hasan as) was ill as a result of remembering the martyrdom of Al Zahraa (pbuh) one year and he said:
“It is an illness that befalls me at the time of the afflictions of Aal Muhammad (pbut), and it is from the mercies of Allah, praise be to Allah, Lord of the Worlds”
I said: there is no ability nor strength but with Allah, the Most High, the Mighty, praise be to Allah who has made murder a thing of habit for you, and martyrdom for you is a thing of honour from Allah, may Allah make your reward great, my master, and recompense you with every sound of pain and all sadness:
He said, “We are for Allah, and we will return to Him. The Trust has been returned, and the Pledge has been taken back, and Al Zahraa has been robbed, so how ugly to me is this world (lit. the greenery and the dust) Oh Messenger of Allah”.
With the Pious Servant vol. 1

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في ذكرى شهادة الزهراء (ع)..
كان (ع) مريضاً بسبب ذكرى شهادة الزهراء (ع) في أحد الأعوام، فقال:
(هو مرض يصيبني في بعض مصائب آل محمد (ع) وهو من نعم الله، الحمد لله رب العالمين)
فقلت: لا حول ولا قوة إلا بالله العلي العظيم، الحمد لله وقد جعل القتل لكم عادة وكرامتكم من الله الشهادة، عظم الله لك الأجر مولاي وأجزل لك العطاء بكل آهة وحسرة. فقال (ع): هذه كلمة لأمير المؤمنين (ع) , لم تمرّ عليّ يوماً إلا وأبكتني أنقلها لك لعلك تنتفع بها: “إنا لله وإنا إليه راجعون، قد استرجعت الوديعة، وأخذت الرهينة، واختلست الزهراء، فما أقبح الخضراء والغبراء يا رسول الله”
مع العبد الصالح-ج1