Ibn Majah

Commentary:

(1)
The wisdom behind the prohibition of praying where a grave is in front is that, outwardly, it should not appear as if one is prostrating to the grave.
Even if there is no intention of prostrating to the grave or its occupant.
In the funeral prayer (salat al-janazah) as well, bowing (ruku) and prostration (sujud) have not been prescribed because the deceased is in front, so that even outwardly it does not take the form of prostration. For this very reason, a person who could not participate in someone’s funeral prayer may later perform the funeral prayer at the grave. See: (Sunan Ibn Majah, Hadith: 1533, 1527)

(2)
Some people build a mosque near the grave of a prophet or a saint (wali) and think that praying here is superior due to the blessings of the buried personality, whereas this too is legally prohibited, even if the grave is not in front while praying.
The reason for this is that building places of worship over the graves of elders and prophets is the custom of the Jews and Christians.
The Messenger of Allah (sallallahu alayhi wa sallam) strictly forbade this.
The Prophetic statement is:
“Allah’s curse be upon the Jews and Christians who took the graves of their prophets as places of prostration.” (Sahih al-Bukhari, al-Salat, Hadith: 436, 435)
And Sahih Muslim, al-Masajid, Chapter: The prohibition of building mosques over graves.
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..., Hadith: 529)

(3)
Some people present the hadith that the grave of Isma’il (alayhis salam) is in Hatim as evidence for the permissibility of a grave in a mosque, whereas that is part of the Ka’bah.
Similarly, in the Prophet’s Mosque (Masjid al-Nabawi), the graves of the Prophet (sallallahu alayhi wa sallam), Abu Bakr, and Umar (radi Allahu anhuma) are present.
This argument is not valid because, even if it is established that the grave of Isma’il (alayhis salam) is in Hatim, its sign has been erased, so it no longer holds the ruling of a grave.
And the graves of the Prophet (sallallahu alayhi wa sallam) and the two Shaykhs (radi Allahu anhuma) were built outside the Prophet’s Mosque.
Neither Allah nor His Messenger (sallallahu alayhi wa sallam) nor the Companions (radi Allahu anhum) ordered for them to be included in the mosque.
Wrong actions of later generations cannot serve as evidence for any legal ruling.
Moreover, the blessed chamber (hujrah mubarakah) of A’ishah (radi Allahu anha), in which these graves are present, is closed from all sides and it is not possible to enter there, so in this way, it is as if it has been separated from the mosque.
Despite this, cautious scholars still say that if the rulers of the present age were to separate this section from the mosque by means of a wall, so that the passage to it would be completely separate, it would be much better.

1:
That is, "pray wherever you wish." From this hadith, it is understood that performing prayer in graveyards and bathhouses (hammam) is not permissible.
As for the bathhouse (hammam), the reason is that there is a suspicion of impurity and filth being present there. In the case of the graveyard, the prohibition is as a preventative measure (sadd al-dhara'i) to safeguard against shirk (associating partners with Allah).
In some other hadiths, there is also prohibition regarding performing prayer in certain other places;
their details will be mentioned ahead.

492. Commentary:
➊ Among the mentioned chains of narration, the narration of Musaddad is certainly marfu‘ (attributed directly to the Prophet sallallahu alayhi wa sallam). However, in the narration of ‘Amr ibn Yahya, there is conjecture; it is not certain. The noble hadith scholars were extremely sensitive and cautious in transmitting the statements of the Messenger (sallallahu alayhi wa sallam).

➋ Qadi Abu Bakr Ibn al-‘Arabi rahimahullah states that there are thirteen places where prayer is not performed: a garbage dump, a slaughterhouse, a graveyard, the middle of a pathway, a bathhouse (hammam), a camel pen, the roof of the Ka‘bah (Baytullah), facing the direction of graves, towards the wall of a toilet when there is impurity on it, the places of worship of Jews and Christians, facing idols and pictures, a place of punishment, and al-‘Iraqi added further: on usurped land, in Masjid Dirar, and the place where an oven (tannur) is in front. For details, see: [نيل الأوطار، باب المواضع المنهي عنها والمازون فيها لصلوة 55/2]

Lexical Explanation:
«وَلَه عِلَّةٌ» "And there is a reason (‘illah) in it." That reason is that Hammad narrated it as a connected (mawsul) narration, whereas Imam Thawri rahimahullah narrated it as a disconnected (mursal) narration. Imam Daraqutni rahimahullah and Bayhaqi rahimahullah have said that this hadith is preserved only with a mursal chain. However, some scholars have considered its being mawsul (connected) as authentic, and this is the preferred view. This is also supported by other ahadith.

Benefit:
From this hadith, it is understood that prayer (salah) is not valid in graveyards and bathhouses (hammam). In the bathhouse, this is because the place is impure, and in the graveyard, the prohibition is as a preventive measure (sadd al-dhara’i) to avoid falling into shirk (associating partners with Allah).