Bukhari

Hadith Commentary:
Imam Bukhari rahimahullah has mentioned in the chapter heading, in an absolute sense, the fasting at the end of the month, whereas in the hadith, the mention is specifically of the end of the month of Sha'ban. From this, it is understood that according to Imam Bukhari rahimahullah, fasting at the end of any month in general is legislated, so that a person becomes accustomed to it.

Whereas, in another hadith, it is stated that fasting one or two days before Ramadan is prohibited.

This applies in the case when the fasts are observed with the intention of welcoming (istiqbal) Ramadan. If there is no such intention, but rather it is a person's habit, then there is no objection to fasting at the end of Sha'ban.

Hadith Commentary: Benefits and Issues: (1)
The Interval Between the Construction of the Ka‘bah and Masjid al-Aqsa (Bayt al-Maqdis):
The well-known view regarding the construction of the Ka‘bah and Bayt al-Maqdis is that the Ka‘bah was built by Ibrahim (alayhis salam) and Bayt al-Maqdis was constructed by Sulayman (alayhis salam), and the interval between them amounts to more than a thousand years. However, in the hadith, the interval is mentioned as forty years. The actual matter is that Ibrahim (alayhis salam) and Sulayman (alayhis salam) did not lay the foundations (ta’sis) of these mosques, but rather renewed (tajdid) them (i.e., rebuilt them anew). The original construction was done by the angels before the creation of Adam, and the interval between these constructions and formations is forty (years), or what is meant is the construction by Adam (alayhis salam). Adam (alayhis salam) laid the foundations of both mosques, and the interval between them was forty years. And if the construction by Ibrahim is intended, then it is apparent that just as a place of worship was built for one son, Isma‘il, and his progeny, so too a place of worship would have been constructed for the other son, Ishaq, and his progeny. Therefore, the construction of Bayt al-Maqdis referred to here is that of Ya‘qub ibn Ishaq (alayhis salam), and the interval between the two constructions is forty years.

(2)
The meaning of “wherever the time of prayer comes upon you, pray there” is that you should pray wherever the Shari‘ah has not prohibited prayer. Because, just as purity of clothing and body is necessary for prayer, so too is the cleanliness and purity of the place. The Shari‘ah has prohibited praying in graveyards, bathhouses (hammam), slaughterhouses (the place of slaughter), public roads, and places of filth.

Benefits and Issues:
➊ This hadith apparently contradicts the previous hadith in which it is stated that one should not fast a day or two before the start of Ramadan. However, the reconciliation between them is that this concession and emphasis is for the person who has made a vow (nadhar) to fast, or who is accustomed to fasting on specific days from before; such a person should continue to fast as usual. But if someone, without any prior habit or vow, wishes to fast voluntarily as a way of welcoming Ramadan, then it is not permitted.

➋ The person whom the Prophet sallallahu alayhi wa sallam instructed to fast one or two days after Ramadan was someone who used to fast at the end of the month, but he had left off fasting at the end of Sha'ban out of fear that it might fall under the category of welcoming Ramadan, which is prohibited.

➌ The various meanings of the word (sarara) are mentioned after the following narration.