Bukhari

Hadith Commentary:
The meaning of both statements is that those who die upon tawhid (monotheism) will certainly enter Paradise, and those who die upon shirk (polytheism) will remain in Hell forever.
By shirk is meant the worship of graves, shrines, and taziyahs, just as disbelievers worship idols; both types of people are considered mushrik (polytheists) in the sight of Allah.
Even the slightest trace of shirk is a very great sin before Allah.
Therefore, it is necessary for every Muslim to strive to remain far away from shirk.

Hadith Commentary:
It is narrated from Hazrat Jabir radi Allahu anhu that the Messenger of Allah sallallahu alayhi wa sallam said:
"Whoever dies without associating anyone with Allah will enter Paradise, and whoever dies while associating someone with Allah will enter Hell." (Sahih Muslim, Al-Iman, Hadith: 269(93))

When both statements are authentically reported from the Messenger of Allah sallallahu alayhi wa sallam, then why did Hazrat Abdullah bin Mas'ud radi Allahu anhu attribute the last statement to himself? The answer is that perhaps Hazrat Abdullah bin Mas'ud radi Allahu anhu only heard the first statement from the Messenger of Allah sallallahu alayhi wa sallam, or if he did hear it, he forgot it. In both cases, he deduced it himself from the Qur'an and Hadith, that the negation of the cause necessitates the negation of the effect, because when the cause for entering Hell is absent, then there is no place other than Paradise where he can be admitted.

In other words, the entry of a polytheist (mushrik) into Hell necessitates the entry of a believer into Paradise.

This is not an opposing meaning (mafhum mukhalafah), but rather a necessary implication (lazim ma'na).

2.
In any case, the meaning of both statements is that those who die upon tawhid (monotheism) will certainly enter Paradise.

And those who die upon shirk (polytheism) will remain in Hell forever.

By shirk is meant worshipping graves, shrines, and taziyahs, just as the disbelievers used to worship idols; in this way, both types of people are considered polytheists (mushrik) in the sight of Allah.

Even the slightest shirk is a very great sin and an unforgivable crime, therefore a Muslim should adhere firmly to tawhid and keep himself far from shirk.

Wallahu al-musta'an (And Allah is the One whose help is sought).

Hadith Commentary:
Further clarification of this is found in the hadith of Anas (radi Allahu anhu), in which Allah the Exalted said:
O son of Adam! If you come to Me with sins filling the whole world, but you did not associate partners with Me, I will come to you with forgiveness filling the whole world.
(Related by al-Tirmidhi)
In summary, shirk (associating partners with Allah) is the gravest sin, and tawhid (affirming the oneness of Allah) is the greatest virtue.
A sinful monotheist is, in every respect, a thousand times better than a devout polytheist.

Hadith Commentary:
(1)
Imam Bukhari rahimahullah has mentioned a marfu‘ (attributed to the Prophet) hadith in the chapter heading. It is narrated from Mu‘adh ibn Jabal radi Allahu anhu, who said: The Messenger of Allah sallallahu alayhi wa sallam said: “Whoever’s last words are la ilaha illallah will enter Paradise.” (Sunan Abi Dawud, Al-Jana’iz, Hadith 3116)

By this chapter heading, Imam Bukhari rahimahullah intends to convey that whoever says la ilaha illallah and dies upon it, and does not associate anyone as a partner with Allah, will enter Paradise.

It is also possible that Imam Bukhari rahimahullah means that whoever says la ilaha illallah with sincerity at the time of death, by doing so his previous sins will be forgiven and he will enter Paradise, because sincerity necessitates repentance and remorse.

It can also mean that even if a person has committed adultery or violated the rights of others (such as theft), in such cases, Allah Ta‘ala will certainly create a way for the fulfillment of the rights of others.

And Allah knows best.

(2)
Through the statement of Ibn Mas‘ud radi Allahu anhu, Imam Bukhari rahimahullah wants to clarify that it is not necessary to recite the word of sincerity (kalimat al-ikhlas) at the time of death in order to enter Paradise; rather, what is meant is to hold the belief in tawhid (the oneness of Allah) and to die upon that belief.

In our view, from the established chapter heading and the presented ahadith, Imam Bukhari rahimahullah’s objective is that the sick and those near death should be prompted to recite the kalimah tayyibah (the pure word), so that their end may be upon faith.

Although there are clear ahadith regarding prompting (talqin) in the books of hadith, they did not meet Imam Bukhari rahimahullah’s conditions, and thus were not presented. For example, in Sahih Muslim it is stated: Prompt your dying relatives to say la ilaha illallah. (Sahih Muslim, Al-Jana’iz, Hadith: 2123 (916))

Prompting does not mean that the testimony of faith (shahadatayn) should merely be mentioned in his presence, but rather that he should be told to recite it, as when the Messenger of Allah sallallahu alayhi wa sallam went to visit an Ansari who was ill, he said to him: Say la ilaha illallah. (Musnad Ahmad: 152/3)

Hadith Commentary: Benefits and Issues:
Tawhid (monotheism) is the essence, spirit, and core of the entire religion. In other words, it is the title of the whole religion, because tawhid is that no one is associated with Allah in His essence, attributes, names, actions, or His rights. And the one who consciously affirms this tawhid cannot, in any aspect of life governed by religion and Shariah, act in opposition to it; therefore, he will enter Paradise directly.

In contrast, shirk (associating partners with Allah) is to associate anyone with Allah in His essence, or His attributes and names, or His actions, or His rights. Whoever commits any part of shirk will not enter Paradise directly. However, if he possesses any part of tawhid, then on the basis of that, after undergoing some punishment, he may eventually enter Paradise.

In such ahadith, the consequences of true tawhid and actual shirk are described, in which there is no mixing of one with the other, because the properties of individual elements change when they are combined with something else.