Bukhari

Hadith Commentary:

Ibn Battal said that from this it is derived that i'tikaf (spiritual retreat in the mosque) is a confirmed Sunnah (sunnah mu'akkadah). And Ibn al-Mundhir narrated from Ibn Shihab that he expressed amazement at the Muslims for abandoning i'tikaf, even though the Messenger of Allah (sallallahu alayhi wa sallam), from the time he arrived in Madinah until his passing, never abandoned i'tikaf.

This year, the Prophet (sallallahu alayhi wa sallam) performed i'tikaf for twenty days because he had come to know that his death was near.

Hadith Commentary:
(1)
Although the last ten days (asharah) are superior for i‘tikaf (spiritual retreat), because Laylat al-Qadr (the Night of Decree) falls in its odd nights, it is not obligatory; i‘tikaf can also be observed before this period. In the last year of his life, the Messenger of Allah (sallallahu alayhi wa sallam) observed i‘tikaf for twenty days. The hadith scholars (muhaddithin) have mentioned several reasons for this:
➊ Since he (sallallahu alayhi wa sallam) had been informed of his impending death, he observed i‘tikaf for twenty days to teach the ummah that in the final part of one’s life, one should exert more effort in acts of worship.
➋ Jibril (alayhis salam) used to review the Qur’an with the Messenger of Allah (sallallahu alayhi wa sallam) once every year, but in the year of his passing, he reviewed it twice; therefore, the Prophet (sallallahu alayhi wa sallam) also increased the duration of his i‘tikaf.
➌ The Messenger of Allah (sallallahu alayhi wa sallam) was unable to observe i‘tikaf one year due to travel, so he performed i‘tikaf for twenty days to make up for it.
➍ Because of the tents of his wives being set up, he (sallallahu alayhi wa sallam) abandoned i‘tikaf in Ramadan, although he made up for it in the month of Shawwal; however, to further solidify this, he observed i‘tikaf for twenty days the following year.
It is also possible that, due to various reasons, he (sallallahu alayhi wa sallam) observed i‘tikaf for twenty days on multiple occasions—that is, once due to travel and another time due to the review of the Qur’an.
(Fath al-Bari: 4/362)
However, the wording of the hadith does not support multiplicity (of such occurrences).
And Allah knows best.
(2)
The example of the person observing i‘tikaf is like that of a slave who throws himself before his master and says, “I will not leave this place until you forgive me and pardon my sins.”
(‘Umdat al-Qari: 8/289)

Benefits and Issues:
It is understood that i'tikaf can also be observed in the middle of Ramadan.
Perhaps the Prophet sallallahu alayhi wa sallam had come to know of the nearness of his death, which is why he became extremely eager in worship.
In this Ramadan, Jibril Amin alayhis salam also reviewed the Qur'an with him twice.

Benefits and Issues: ➊
By “presenting the Qur’an” is meant reviewing the Qur’an. Angel Jibril (alayhis salam) would review with the Messenger of Allah (sallallahu alayhi wa sallam) every year in Ramadan whatever portion of the Qur’an had been revealed up to that point. (Sahih al-Bukhari, Book of Fasting, Chapter: The Generosity of the Prophet (sallallahu alayhi wa sallam) in Ramadan, Hadith: 1902)

➋
One possible reason for performing i‘tikaf (spiritual retreat) for twenty days in the last year is that the Messenger of Allah (sallallahu alayhi wa sallam) exerted himself even more in worship during the final part of his life, and since i‘tikaf is also a form of worship, he increased it as well. It is also possible that one ten-day period was to make up for the i‘tikaf missed in the year of the conquest of Makkah, because the expedition of the conquest of Makkah occurred in Ramadan of the 8th year after Hijrah. The Messenger of Allah (sallallahu alayhi wa sallam) entered Makkah victoriously on the 17th of Ramadan and stayed in Makkah Mukarramah for nineteen days, so i‘tikaf could not be performed that year. Thus, in Ramadan of the 10th year after Hijrah, he performed i‘tikaf for twenty days. And Allah knows best.