Calamity of Thursday
Imam Ahmad bin Hanbal narrates via his own chain from Jabir bin Abdullah Ansari who said: "The Prophet [pbuh] on his deathbed asked for a Paper to write something so that that they (Ummah) will not deviate after his death. Umar bin Khattab opposed him and he abandoned writing it."
Researcher of this book Shaykh Shou'ib AlArna'ut says : sahih lighairihi.
[Ref : Musnad Ahmad bin Hanbal - Imam Ahmad bin Hanbal // Vol 13 // Page 68 // Hadith 14726 // Edn. Risalah Alamiyyah Beriut] 
Who was the person that called the Prophet(pbuh) delirious?
Ibn Taymiyyah confirms it was Umar who accused Rasulallah (ﷺ) of being delirious:
As for ‘Umar, he suspected whether the Prophet’s saying was due to the severity of his illness, or was it from his well known sayings. Which is why he(‘Umar) said: “What is wrong with him? Is he delirious?”
[Minhāj al-Sunnāh al-Nabawīya by Ibn Taymīyyah] 
And from the hadith of the Prophet’s illness, he said: “What is his affair? Is he delirious?” Either from obscenity or delirium. And the one who said it was ‘Umar”
[Kitāb al-Nihaya Fī Gharīb al-Hadīth wal-Āthar by Ibn al-Atheer, volume 5, page 246] 
As stated in the sahīh ahadīth, that the Prophet said in his illness: “Bring me an inkwell, I will write for you a book after which you will never go astray. ‘Umar said: “That the man is babbling, so the book of God is sufficient for us”
[Nasīm al-Rīyādh Fī Sharh Shifa’ al-Qādhi ‘Ayadh volume 1, page 411] 
From his sermon on the day of Ghadīr Khum with the agreement of everyone and he says: “Whom I was his Mawla(master), then ‘Alī is his Mawla(master)”, so ‘Umar said: “Bakhn Bakhn, Oh Abā al-Hassan, for you have become my Mawla and the Mawla of every Mawla.” This is a hand over, a response and an arbitration. Then after this, the passion for the love of the leadership prevailed, then he carried the pillar of the caliphate; the contracts of prophecy, the throbbing of passion in the clatter of the banners and the clash of crowded horses and the conquest of the world, whom’s cup goaded them, and they returned to the first disagreement, and they took it behind their backs and bought in it with small price. And when the messenger of God died, he said before his death: “Bring an inkwell, to remove the problem for you and to mention who is entitled after me. ‘Umar said: “Leave this man for he is delirious, it was said he was blabbing”
[Majmū’at Rasā’il by Imām al-Ghazāli page 483] 
Many of our opponents claim that the reason ‘Umar opposed the Prophet (ﷺ) and prevented him from writing the book was because ‘Umar believed the Prophet (ﷺ) was ill and would write something he did not intend to. Let’s see what the Sunnī ahadīth say:
He says “And the occurrence of that(meaning the delirium of the Prophet) is impossible.. because he is infallible in the event of his illness and health, for the Almighty says: “Nor does he speak from [his own] inclination.”
And for the Prophet’s saying: “I don’t say in the anger and satisfaction except the truth”
[Ref: Fath al-Bari with the explanation of Sahih al-Bukhari by Al-Asqalani part 8 page 4566] 
On the authority of Abdullah ibn Umar, he said “I used to write everything i heard from the Prophet (ﷺ), I want to save it, so the Quraysh forbade me, so they said: You write everything you hear from the Prophet (ﷺ) and the messenger of Allah is a human, he speaks in the anger and satisfaction. So I stopped writing, then i mentioned that to the messenger of Allah. So he said: “Write and by whom my soul is in his hand, what came out of me is nothing but the truth”
(The chain of narrators is authentic)
[Ref: Musnad Ahmad ibn Hanbal volume 11 page 57/58] 
“It isn’t appropriate for the Prophet (ﷺ) to delirium or to say any talking is not exact in any of his cases, peace be upon him, however all that he speaks is right and true, and there is no oversight, in-caution or error, in the event of his health and illness, his sleep and wakefulness, his anger and satisfaction.
[Ref: Mashareq al-Ansar by Al-Maliki, volume 2, page 448] 
Al-Nawawi says: “Know that the Prophet (ﷺ) is infallible from lying and from changing some of the islamic rules if he is healthy or ill.
[Ref: Sahih Muslim Sharh al-Nawawi volume 11 page 2017] 
Subhanallah. So it is impossible for the Prophet (ﷺ) to be in a state of delirium, and whatever comes out of the Prophet (ﷺ) is nothing but the truth. So there is no valid reason as to why ‘Umar opposed the Prophet (ﷺ).
Were these words from the Prophet (ﷺ) a revelation or was he reminding a specific group of people who did reject a past revelation?
Ibn Abbās states:
قَرِّبُوا يَكْتُبْ لَكُمُ النَّبِ - Come so that the Prophet would write for you
وَبَيْنَ أَنْ يَكْتُبَ لَهُمْ - He writes to them. He didn’t say “Come so that the Prophet would write for us”, rather he said, “Come so that the Prophet would write for you” He didn’t say “He write to us“, rather he said, “He write to them”
This is not a new revelation by the Prophet (ﷺ) , or Ibn Abbās would say “Come so that the Prophet would write for us” and “He writes to us“. So we know that the Prophet (ﷺ) is speaking about something that has already been revealed in the past, and that it is aimed for a certain group who did not accept/acknowledge this revelation.
Narrated Ibn ‘Abbas:
When Allah's Messenger (ﷺ) was on his death-bed and in the house there were some people among whom was `Umar bin Al-Khattab, the Prophet (ﷺ) said, "Come, let me write for you a statement after which you will not go astray." `Umar said, "The Prophet (ﷺ) is seriously ill and you have the Qur'an; so the Book of Allah is enough for us." The people present in the house differed and quarrelled. Some said "Go near so that the Prophet (ﷺ) may write for you a statement after which you will not go astray," while the others said as `Umar said. When they caused a hue and cry before the Prophet, Allah's Messenger (ﷺ) said, "Go away!" Narrated 'Ubaidullah: Ibn `Abbas used to say, "It was very unfortunate that Allah's Messenger (ﷺ) was prevented from writing that statement for them because of their disagreement and noise."
[Sahih Bukhari 5669] 
It is a confirmation of what the Prophet (ﷺ) said on the day of Ghadeer:
هَلُمَّ أَكْتُبْ لَكُمْ كِتَابًا لاَ تَضِلُّوا بَعْدَهُ - Bring me, so I will write you a book, you will never go astray after it.
The Prophet (ﷺ) said the same thing in a different occasion:
إِنِّي تَارِكٌ فِيكُمْ مَا إِنْ تَمَسَّكْتُمْ بِهِ لَنْ تَضِلُّوا بَعْدِي - Indeed, I am leaving among you, that which if you hold fast to them, you will never go astray after me. One of them is greater than the other: The Book of Allah is a rope extended from the sky to the earth, and my family - the people of my house - and they shall not split until they meet at the Hawd, so look at how you deal with them after me.
What would’ve been written in the book that the Prophet (ﷺ) wanted to write?
Talking about the book which the Prophet wanted to write on Thursday
“(In the book) It was said that he would appoint the caliphs after him”
[Fatḥ al-Bārī bi Sharḥ Ṣaḥīḥ al-Bukhārī by Ibn Hajar al Asqalani part 8 page 366] 
He says: “The difference was in what was meant by the book. It was said he wanted to write a book on rulings so that the difference would disappear. Rather, he wanted to state the names of the caliphs after him so that there would be no differences between them”
[Fatḥ al-Bārī bi Sharḥ Ṣaḥīḥ al-Bukhārī by Ibn Hajar al Asqalani volume 1 page 218] 
He says, “I write you a book”: It was said that he wanted to provide the caliphate in a certain person so that conflict and strife would not occur”
[al-Dibaj ala Sahih Muslim ibn al-Hajjaj by Al-Suyuti volume 4 page 231] 
“The scholars differed as to what he wanted to write for them on the two sides. The first is that he wanted to state the caliph after him. The second is to write a book that provides rules to remove disagreement. And the first is the show up”
[Kashf al-Mushkil li ibn al -Jawzī min Hadīth al-Sahīhayn volume 4, page 214] 
“This meaning, which the Prophet intended to write, is likely to be a detail of important matters that occurred in the Sharī’a, so he wanted to define them. And it is possible that he wants to make a statement about what they’re going to refer to when fitna happens, and who the first to be followed and be pledged allegiance to is. And it is possible that he wanted to clarify the matter of the caliphate, and to appoint the caliph after him. And this is the closest.
[Kitāb al-Mufhim li-mā ashkala min Talkhīṣ Ṣaḥīḥ Muslim by Qurṭubī volume 4 page 558] 
So now the opponents are given two choices.
1. The Prophet didn’t write the book and left them in conflict.
2. Or the book was written in the past but the hypocrites did not acknowledge it.
Who are these caliphs that the Prophet (ﷺ) wanted to write down?
On the authority of Zayd ibn Thābit, from the Prophet (ﷺ), he said: “I leave behind for you two caliphs, the Qur’ān and my Ahlulbayt, and they will not separate until they return to me in Al Hawdh.”
Narrated by Al-Tabarāni in Al-Kabeer and the rijal is thiqah(trustworthy)
[Majmu’ al-Zawā’id by Al Haythami, volume 1, page 230] 
From Zayd ibn Thābit, who said: The Prophet (ﷺ) said: “I leave behind for you two caliphs, the Qur’ān, an elongated rope between heaven and earth. And my Ahlulbayt, and they will not separate until they return to me in Al Hawdh.”
Isnad is Hasan
[Musnad Ahmad Ibn Hanbal volume 13, page 28] 
“I have left for you two caliphs: The Qur’ān, an elongated rope between heaven and earth, and my Ahlulbayt, and they will not be separated until they return to me in Al-Hawdh. I forsake you as soon as you hold on to them, you will never go astray after me, one is greater than the other”
Isnad is Sahih
[Sahih Al-Jami’ Al-Saghir by Al-Albāni, volume 1, page 482] 
The Prophet said: “I leave for you two caliphs, the Qur’ān, an elongated rope from Heaven to Earth and my Ahlulbayt...”
“...(I leave for you) after my death (two caliphs)...” “...(the Qur’ān) (and my offspring) and they are the people of the Kisa’, which Allāh has taken away the rijs and purified them.”
[Fayidh al-Qadīr Sharh al-Jami’a al-Sagheer by Mannawi, volume 3, page 14]