Bukhari

Hadith Commentary:
In this hadith, a principle has been stated regarding the difference in reward between performing prayer while standing, sitting, and lying down.
As for the issue of whether performing prayer while lying down is even permissible or not, this is not discussed here; therefore, it cannot be questioned regarding this hadith that, if prayer while lying down is not permissible, then how is its reward mentioned in the hadith? The author (rahimahullah) has also titled these hadiths with the purpose of clarifying this very principle.
The details of this are established from the Lawgiver (Shari‘) himself at other occasions.
Therefore, in practical terms, the decision regarding permissibility or non-permissibility will be made in light of those details.
Discussion on the first two hadiths of this chapter has already passed, that the Messenger of Allah (sallallahu alayhi wa sallam), due to an excuse, could not go to the mosque, so he performed the obligatory prayer at his residence.
After completing the prayer, the Companions (radi Allahu anhum) came to visit him, and when they saw him praying, they also made the intention to follow him in prayer.
The Companions (radi Allahu anhum) were praying while standing, so he (sallallahu alayhi wa sallam) forbade them, as in voluntary (nafl) prayer, it is not appropriate for the followers (muqtadis) to stand in opposition to the state of the imam in this manner.
(Tafheem al-Bukhari, vol. 5, p. 13)
A sick person who cannot pray even while sitting may pray while lying down.
There is no doubt regarding its permissibility.
The ruling that followers should pray sitting with the imam was later abrogated.

Hadith Commentary:
(1)
In this hadith, it is not clarified whether ‘Imran bin Husayn radi Allahu anhu asked about performing the obligatory prayer while sitting or about the voluntary (nafl) prayer.
Hafiz Ibn Hajar rahimahullah writes:
If a person, despite hardship, performs the obligatory prayer standing, then this is more virtuous and preferable. However, if someone, under such circumstances, performs the prayer sitting, he will receive the reward of a healthy and able person.
As mentioned in the hadith:
“Whoever falls ill or is on a journey, then whatever good deeds he used to do while healthy and resident, the same reward will be written for him in those circumstances.”
(Fath al-Bari: 2/755)
If one performs a voluntary (nafl) prayer sitting, even though he has the ability to stand, then such a person will receive half the reward. But if he performs the nafl prayer sitting due to helplessness and inability, then he will receive the full reward. However, if the obligatory prayer is performed sitting despite having the ability to stand, then this is not correct.
(‘Umdat al-Qari: 2/433)

(2)
From the generality of performing prayer while sitting, the Messenger of Allah sallallahu alayhi wa sallam is an exception, because it is his special distinction that even if he performs a voluntary (nafl) prayer sitting, Allah grants him the full reward. As in Sahih Muslim, ‘Abdullah bin ‘Amr radi Allahu anhu says:
I learned from the Prophet’s instruction:
The one who prays sitting receives half the reward.
One day, I saw the Messenger of Allah sallallahu alayhi wa sallam praying while sitting, so out of astonishment, I placed my hands on my head.
The Messenger of Allah sallallahu alayhi wa sallam asked me, and I explained the reason for my astonishment.
Upon this, he said: I am not like you.
(Sahih Muslim, Salat al-Musafirin, Hadith: 1715(735))

(3)
It seems appropriate at this point to present a summary of the research of the renowned scholar of the Arab lands, Fadilat al-Shaykh Muhammad bin Salih al-‘Uthaymin rahimahullah. He states:
It is necessary for the sick person to perform the prayer standing, even if he stands with a slant or by supporting himself against a wall, pillar, or staff.
If in no way is he able to stand, then he is permitted to pray sitting, but in this case, it is better that during the standing and bowing (ruku‘) positions, he sits cross-legged, and during prostration (sujud), he sits on his shins (two knees).
(Salat al-Marid, p. 8)

Hadith Commentary:
(1)
There is no mention in the hadith of performing prayer by indication (isharah).
Since the hadith does not specify in detail how a person praying while sitting should perform the prayer, in light of this generality, it appears that Imam Bukhari rahimahullah inclines to the view that if a person praying while sitting performs bowing (ruku) and prostration (sujud) by indication, it is permissible for him.
Some Maliki scholars hold the position that even if a person praying while sitting is able to perform bowing and prostration, he is still permitted to perform the prayer by indication.
And Allah knows best.

(2)
Shaykh Muhammad ibn Salih al-Uthaymin rahimahullah writes:
If a sick person cannot pray while sitting, then he should lie on his side and pray facing the qiblah, and lying on the right side is preferable.
If he is unable to face the qiblah, then he should pray facing whichever direction he is able.
If he cannot lie on his side, then he should pray lying on his back.
In this case, his feet will be towards the qiblah.
It is better that he place something under his head to elevate it so that his face can be towards the qiblah.
If his feet cannot be towards the qiblah, then he should pray in whatever way is possible.
The sick person should perform bowing (ruku) and prostration (sujud).
If he is unable, then he should perform bowing and prostration by indication (isharah).
In such a state, he should incline more during prostration than in bowing.
If he cannot pray by indicating with his head, then he should pray by indicating with his eyes.
He should close his eyes slightly during bowing and close them more during prostration.
Some patients pray by indicating with their finger; there is no evidence for this in the Book or the Sunnah, nor has any scholar permitted it.
If he cannot even indicate with his eyes, then he should perform the prayer in his heart, that is, he should intend bowing, prostration, standing, and sitting in his heart. Every person will have only what he intended. In any case, as long as he is conscious, prayer is not waived from him.
(Salat al-Marid, p. 6)

1:
And this statement pertains to voluntary (nafl) prayers, as will be mentioned further ahead.

951. Commentary:
➊ If someone is ill or weak and cannot stand, then by performing the prayer while sitting, insha Allah, he will receive the full reward.
➋ It is absolutely impermissible to perform obligatory (fard) prayers while sitting or lying down without a valid excuse, despite having the ability. [عون المعبود]
However, performing voluntary (nafl) prayers while sitting without an excuse results in half the reward being diminished.

1661. Commentary: From the mentioned hadith, it is understood that voluntary (nafl) prayers can be performed while sitting or lying down without any excuse. However, the one who prays sitting will receive half the reward of the one who prays standing, and the one who prays lying down will receive half the reward of the one who prays sitting. Nevertheless, if there is an excuse, the full reward will be granted.

Benefits and Issues:
➊ Without a valid excuse, performing the prayer while sitting or lying down results in a reduction of reward.

➋ The reward for performing the prayer while lying down is even less than that for performing it while sitting; therefore, one should avoid performing the prayer while sitting or lying down without a valid excuse.