Bukhari

Hadith Commentary:
1.
In some manuscripts of Sahih Bukhari, the words "mada al-rajul" appear with the chapter heading, while in others they do not.
In our copy, these words are present; therefore, with this heading, Imam Bukhari rahimahullah intends to clarify the legal ruling regarding lying on one's back and stretching out one's legs in the mosque. It is evident that the mosque is a place for worship, not for sleeping and lying down, and in some hadiths, this has also been prohibited.
As is found in Sahih Muslim, the Messenger of Allah sallallahu alayhi wa sallam said:
"None of you should lie on his back and then do this:
that he places one of his legs over the other."
(Sahih Muslim, Kitab al-Libas, Hadith: 5503 (2099))
So regarding this prohibition, it will be said that its context and circumstance are different.
This applies in a situation where it is not possible to fully observe the covering of the awrah (private parts), for example:
if the garment is short and by lying in this manner there is a risk of exposing the awrah.
It should also be clarified that at that time in Arab society, it was customary to wear an izar (waist-wrapper), and trousers and the like were not worn; however, if the garment is ample and there is no risk of exposing the awrah, rather, full care has been taken to cover the awrah, then there is no harm in lying on one's back in this manner.
Besides this, it is also worth considering that the Messenger of Allah sallallahu alayhi wa sallam would not lie in this manner in the presence of the general public, but would have done so at times of special rest when ordinary people were not present, because the details of the dignity and seriousness with which the Messenger of Allah sallallahu alayhi wa sallam would sit before all people are also mentioned in the hadiths.
As for the possibility that lying in this manner was not exclusive to the Messenger of Allah sallallahu alayhi wa sallam, Imam Bukhari rahimahullah has also presented the practice of the Rightly Guided Caliphs, namely Umar radi Allahu anhu and Uthman radi Allahu anhu.
It is thus known that lying in this manner in the mosque is permissible not only for the Messenger of Allah sallallahu alayhi wa sallam but for others as well.
(Fath al-Bari: 1/729)
Imam Humaydi rahimahullah has also included the name of Abu Bakr al-Siddiq radi Allahu anhu in this list, stating that he also practiced this.
('Umdat al-Qari: 3/540)
2.
Shah Waliullah Muhaddith Dehlawi rahimahullah writes in Sharh Tarajim Bukhari that in this chapter heading, the permissibility of two things is being presented: lying on one's back and lying with one leg placed over the other.
In other hadiths, this has also been prohibited, so by way of reconciliation it will be said that the hadiths containing prohibition are either abrogated, or the prohibition applies when the izar (waist-wrapper) is short and there is a risk of exposing the awrah.

Takhrij al-Hadith:
[وأخرجه البخاري 475، ومسلم 210، من حديث مالك به]

Jurisprudential Points
➊ It is permissible to lie down and sleep in the mosque.
➋ Both Sayyiduna Umar ibn al-Khattab radi Allahu anhu and Sayyiduna Uthman ibn Affan radi Allahu anhu used to lie down in the mosque with one leg placed over the other. [صحيح بخاري : 475 والموطأ 1/173 ح418]
➌ In an authentic hadith, it is narrated from Sayyiduna Jabir ibn Abdullah al-Ansari radi Allahu anhu that the Prophet sallallahu alayhi wa sallam forbade lying down with one leg placed over the other. See: [صحيح مسلم : 72/2099 [5501]]
This prohibition applies in the case where the person lying down has wrapped himself in a sheet as an izar (lower garment) and there is a fear of the private parts becoming exposed. If a person is wearing trousers or there is no fear of the private parts being exposed, then this prohibition does not apply.

Benefit:
It is permissible to lie down or sleep in the mosque, but the manner of lying down should be such that the private parts and similar areas remain properly covered.