Bukhari

Hadith Commentary:
(1)
In one narration, it is explicitly stated that Allah, the Exalted, has forgiven all the previous and future sins of the Prophet sallallahu alayhi wa sallam, so then why do you endure so much hardship? After this, the Prophet sallallahu alayhi wa sallam gave the answer that is mentioned in this hadith.
(Sahih al-Bukhari, al-Tafsir, Hadith: 4836) (2)
The relevance of this hadith to the chapter heading is that giving thanks to Allah, the Exalted, is obligatory, and abandoning an obligation is forbidden.
When a person occupies himself with fulfilling what is obligatory, he will necessarily refrain from forbidden things.
In any case, patience is essential for gratitude, because through it the servant remains steadfast in obedience to Allah, the Exalted, and refrains from His disobedience.
(Fath al-Bari: 11/369)
And Allah knows best.

Hadith Commentary:
During the early period of the revelation of Surah Al-Muzzammil, this was the practice of the Prophet sallallahu alayhi wa sallam: he would spend most parts of the night engaged in worship.

Hadith Commentary:
(1)
It is understood from a hadith that Aisha (radi Allahu anha) said to the Prophet (sallallahu alayhi wa sallam): "When Allah the Exalted has forgiven all your sins, then why do you subject yourself to such hardship?" He replied:
"I do this as an expression of gratitude to Allah."
From this, it is understood that one way of showing gratitude is through prayer (salah). It is also evident that gratitude should be expressed not only verbally but also through action, because acknowledging a favor with the tongue and fulfilling duties in response is called gratitude (shukr).
(Fath al-Bari: 21/3) (2)
It is understood from this hadith that in matters of worship, one may impose hardship upon oneself, even if it results in physical discomfort. For when the Messenger of Allah (sallallahu alayhi wa sallam) would worship to such an extent, despite knowing that all his sins had been forgiven, then a person who does not know his own end is even more deserving of performing such acts.
This applies in the case where the abundance of worship and the hardship involved do not lead to constriction of the heart, because it is mentioned in the hadith that you should perform deeds according to your capacity, for Allah the Exalted is not tight-fisted in giving reward; rather, it is you who become weary of worship and abandon it.
(Sahih al-Bukhari, al-Libas, Hadith: 5861)

Hadith Commentary: Benefits and Issues: The application of the word "sin" (dhamb) is very broad. Any action that is beneath someone's dignity or status, or is contrary to what is preferable (khilaf awla), is also called a "sin" (dhamb), and beyond that, its application even extends to disbelief and polytheism (kufr and shirk). Here, what is meant are those actions that were beneath your lofty and exalted status, or were contrary to what was preferable. And according to Qadi Sulaiman rahimahullah, what is meant are those accusations that were leveled against you before and after the migration (hijrah). For details, see Rahmatul-lil-Alameen. Pir Karam Shah has transmitted this meaning, but did not mention the name of Qadi Sahib. (Zia-ul-Qur’an, vol. 5, pp. 532–533)

To interpret it this way is not free from affectation, because the incident of Musa alayhis salam—“And they have a sin against me, so I fear that they will kill me” (ash-Shu‘ara: 14) —has been used as evidence, and Musa alayhis salam himself described this sin as wrongdoing (zulm) and sought forgiveness: “He said: ‘My Lord, indeed I have wronged myself, so forgive me.’ So He forgave him. Indeed, He is the Most Forgiving, Most Merciful.” (Surah al-Qasas: 16)

Therefore, the fact that the Prophet sallallahu alayhi wa sallam endured so much hardship in worship was as gratitude for forgiveness, that when Allah Ta‘ala has bestowed such grace and favor upon me that He has forgiven all my sins, then I should express gratitude for this blessing and favor.

From this it is understood that just as gratitude is expressed verbally, it is also expressed through action, as is the statement of Allah Ta‘ala: “Work, O family of Dawud, in gratitude.” (Surah Saba: 13)

English Commentary:
1:
That is, the statement regarding performing voluntary (nafl) prayers for a longer duration.

2:
So when the already forgiven Prophet, sallallahu alayhi wa sallam, used to perform voluntary (nafl) prayers for a long time as an expression of gratitude—that is, he would spend as much time as possible in worship—then we sinful members of his ummah should pay even more attention, through prescribed (masnun) acts, to seeking forgiveness for ourselves and earning as many good deeds as possible.
However, this should be done while avoiding innovations (bid‘ah).

1645. Commentary:

➊ The swelling of the feet of the Messenger of Allah (sallallahu alayhi wa sallam) does not imply laziness, because laziness and vigor are related to the heart and mind.

➋ "Former and latter sins"—this is a hypothetical matter. It has been said that if there was any shortcoming before prophethood. Some have interpreted this as "leaving what is preferable" (tark al-awla), which you would do for the benefit of your ummah, for example: releasing the prisoners of Badr after taking ransom, or performing the funeral prayer for Abdullah bin Ubayy, etc. This has also been described as an ijtihadi error (error in independent judgment).

➌ "A grateful servant"—that is, if Allah the Exalted has suspended accountability from me, then it is also my duty that I remember Him at all times. It is precisely these qualities of the Messenger of Allah (sallallahu alayhi wa sallam) that made you the leader of the first and the last. It is due to the praise of Allah the Exalted that you will be granted the Praiseworthy Station (maqam mahmud). (May my soul and spirit be sacrificed for him, sallallahu alayhi wa sallam.)

➍ Gratitude, just as it is expressed by the tongue, is also expressed through action.

Benefits and Issues:
➊ Prophets are infallible from sin. However, if it is hypothetically assumed that any sin were to occur, then its forgiveness has already been announced beforehand. The purpose of this is to express the lofty status and rank of the Messenger of Allah (sallallahu alayhi wa sallam), or by “sin” may be meant those actions where the Prophet (sallallahu alayhi wa sallam), due to some expediency, left the more virtuous act and chose another permissible act.

➋ When Allah, the Exalted, grants a servant a high rank, he should be even more diligent in expressing gratitude.

➌ The best way to show gratitude is to exert oneself in worship, especially in prayer (salah) and recitation of the Noble Qur’an. Both of these are found in the night prayer (tahajjud).

Benefit:
From this hadith, it is established that the Noble Prophet (sallallahu alayhi wa sallam) used to perform the night prayer (tahajjud) with lengthy standing, bowing (ruku‘), and prostration (sujud). Furthermore, being diligent in tahajjud is, in essence, expressing gratitude to Allah, the Exalted. It is also established from this that the Noble Prophet (sallallahu alayhi wa sallam) was human; therefore, due to prolonged standing, his feet would become swollen.