The First Incident that Led to Dividing the Muslim Ummah into Shi`as and Sunnis

It is the stunningly sad stand taken by Umar ibn al-Khattab and a number of other sahaba against an order by the Messenger of Allah to bring him something to record a testament for them. He promised that this would prevent their backsliding into error.
[It is the famous "Thursday Calamity" recorded in both al-Bukhari's and Muslim's Sahih books]

This Thursday Calamity is, indeed, a most tragic one. It is narrated by all authors of sahihs and sunan and is documented by all traditionists and historians. In a section dealing with the statement of the ailing Messenger (pbuh): "Get away from me," al-Bukhari records it in his Sahih relying on the authority of `Ubaydullah ibn Abdullah ibn `Utbah ibn Mas`ud. Abdullah quotes Ibn `Abbas saying that when death approached the Messenger of Allah, his house became full of men including Umar ibn al-Khattab.

The Messenger of Allah said: "Let me write you something that will forever protect you against straying after me." Umar said: "The Prophet is in a state of delirium, and you have with you the Qur'an; so, the Book of Allah suffices us." Those who were present there argued among themselves, and their argument developed into a dispute. Some of them said: "Come close to the Prophet so that he may write you something that will safeguard you against straying after him," while others repeated what Umar had said. When the argument and dispute intensified in the presence of the Prophet, the Messenger of Allah said to them: "Get away from me." Ibn Abbas used to say: "The calamity, the real calamity, is what discouraged the Messenger of Allah from writing what he wished to write on account of their argument and dispute."

• Al-Bukhari, Sahih, Vol. 4, p. 4

The authenticity of this hadith is not questioned, nor is the occasion whereupon it came to be. Al-Bukhari quotes it in his treatise on knowledge on page 22, Vol. 1, of his Sahih, and it is recorded in many other books. He quotes it in several places of his Sahih. Muslim, too, quotes it at the conclusion of the Prophet's will in his Sahih on page 14, Vol. 2.

Ahmad narrates Ibn Abbas's hadith on p. 325, Vol. 1, of his Musnad. It is narrated by all authors of traditions and books of history, each writer editing it yet retaining its gist, reiterating the fact that the Prophet was described as "hallucinating," or "delirious." But they also mentioned that Umar had said: "The Prophet has been delirious" just to sanitize the statement and undermine the sentiments of those who found it abominable. Supporting this fact is what Abu Bakr Ahmad ibn Abdul-Aziz al-Jawhari has said in his book Al-Saqifa where he relies on the authority of Ibn Abbas.

Ibn Abbas has said,

When death approached the Messenger of Allah, there were many men present at his house. One of them was Umar ibn al-Khattab. The Messenger of Allah said: "Bring me ink and a tablet so that I may write you something that will safeguard you against straying after me." Those present at his house disputed among themselves. Some of them said, "Come close and watch the Prophet write you something," while others repeated what Umar had said. When the argument and dispute intensified, the Messenger of Allah, became crossed and said: "Get away from me."
[Ibn Abul-Hadid, Sharh Nahjul Balagha, Vol. 2, p. 20.]

This proves that the traditionists who did not wish to state the name of the person who went against the Prophet's wish had nontheless quoted his statement verbatim. In a chapter on rewarding the envoys, in his book Al-Jihad wal Siyar, page 118, Vol. 2, al-Bukhari states:

Qabsah narrated a tradition to us from Ibn Ayeenah, Salman al-Ahwal, and Saeed ibn Jubayr. They consecutively quote Ibn Abbas saying: "On a Thursday^__what a day that Thursday was...." He burst sobbing then went on to say, "...the pain of the Messenger of Allah intensified; so, he ordered us to bring him some writing material so that he might write us something whereby we would be protected against straying after him, but people disputed, knowing that nobody should dispute in the presence of any Prophe. They said: `The Messenger of Allah is delirious.' He, therefore, said: `Leave me alone, for the pain I am suffering is more tolerable than what you are attributing to me.' He left in his will three orders: to get the polytheists out of the Arab land, to reward the envoys the same way whereby he used to reward them,' and I forgot the third one."

The same hadith is narrated by Muslim at the conclusion of a chapter dealing with the will in his Sahih, and by Ahmad in Ibn Abbas's ahadith on page 222, Vol. 1, of his work, and by all other traditionists. It is obvious from this incident that Umar ibn al-Khattab was of the view that he was not bound by the Prophet's Sunnah.

This explains the edicts which he issued when he became the caliph and in which he employed his own view even when it contradicted the Prophet's statements. Actually, he followed his own personal views when he contradicted clear divine texts. He thus prohibited what Allah had permitted, and vice versa.  It is only natural to see that all his supporters among the sahaba harbor the same attitude with regard to the Prophet's Sunnah.