Bukhari

Hadith Commentary:
After the passing of the Prophet (sallallahu alayhi wa sallam), some people tried to spread the misconception that the Messenger of Allah (sallallahu alayhi wa sallam), at the time of his death, appointed Ali (radi Allahu anhu) as his executor (wasi).
This matter was also heard by Aisha (radi Allahu anha). Upon this, she said that the last days of the Messenger of Allah (sallallahu alayhi wa sallam) were spent entirely in my chamber.
During those days, I did not leave him alone for even a moment.
At the time of his passing, the Prophet (sallallahu alayhi wa sallam) had his blessed head resting on my chest.
In these circumstances, I do not understand when the Prophet (sallallahu alayhi wa sallam) could have appointed Ali (radi Allahu anhu) as his executor.

Hadith Commentary:
Death was very severe upon the Messenger of Allah (sallallahu alayhi wa sallam). Thus, it is mentioned in the hadith that a vessel of water was present near him; he would repeatedly dip his hand into it and wipe the water over his face again and again.
He said, "La ilaha illallah, truly death has great agonies."
He would also supplicate with these words:
"O Allah! Help me against the agonies of death."
And for enduring the agonies of death, Allah will grant manifold reward. Thus, it is mentioned in the hadith that the group of Prophets (alayhimus-salam) faces great hardships and trials, and therefore our reward will also be multiplied many times over.
(Fath al-Bari: 8/176)

1831. Commentary: Death, in and of itself, is the most painful thing. In comparison to it, other pains are insignificant. The believer is rewarded even for this pain, and sins are forgiven through it; thus, for him, the agony of death becomes a mercy. Whereas for the disbeliever and the hypocrite, it is a punishment. Therefore, the severity or ease of death is not a sign of anyone’s faith or disbelief, or hypocrisy and transgression. Only the person experiencing death knows the severity or pain of it; the onlooker cannot accurately assess it. When the process of death (by the angels) begins, then that person is no longer conscious enough to describe the agony of death. (Astaghfirullah)