The Angels smite the Disbelievers upon capturing Their Souls
Allah says,
وَلَوْ تَرَى إِذْ يَتَوَفَّى الَّذِينَ كَفَرُواْ الْمَليِكَةُ
And if you could see when the angels take away the souls of those who disbelieve (at death);
Allah says, if you witnessed the angels capturing the souls of the disbelievers, you would witness a tremendous, terrible, momentous and awful matter,
يَضْرِبُونَ وُجُوهَهُمْ وَأَدْبَارَهُمْ
they smite their faces and their backs,
saying to them,
وَذُوقُواْ عَذَابَ الْحَرِيقِ
"Taste the punishment of the blazing Fire."
Ibn Jurayj said that Mujahid said that,
وَأَدْبَارَهُمْ
(and their backs), refers to their back sides, as happened on the day of Badr.
Ibn Jurayj also reported from Ibn Abbas,
"When the idolators faced the Muslims (in Badr), the Muslims smote their faces with swords. When they gave flight, the angels smote their rear ends."
Although these Ayat are describing Badr, they are general in the case of every disbeliever. This is why Allah did not make His statement here restrictive to the disbelievers at Badr,
وَلَوْ تَرَى إِذْ يَتَوَفَّى الَّذِينَ كَفَرُواْ الْمَليِكَةُ
يَضْرِبُونَ وُجُوهَهُمْ وَأَدْبَارَهُمْ
And if you could see when the angels take away the souls of those who disbelieve (at death); they smite their faces and their backs,
In Surah Al-Qital (or Muhammad) there is a similar Ayah, as well as in Surah Al-An`am,
وَلَوْ تَرَى إِذِ الظَّـلِمُونَ فِى غَمَرَاتِ الْمَوْتِ وَالْمَلَـيِكَةُ بَاسِطُواْ أَيْدِيهِمْ أَخْرِجُواْ أَنفُسَكُمُ
And if you could but see when the wrongdoers are in the agonies of death, while the angels are stretching forth their hands (saying);"Deliver your souls!" (6;93)
The angels stretch their hands and smite the disbelievers by Allah's command, since their souls refuse to leave their bodies, so they are taken out by force. This occurs when the angels give them the news of torment and Allah's anger.
There is a Hadith narrated from Al-Bara' that;
when the angel of death attends the disbeliever at the time of death, he comes to him in a terrifying and disgusting shape, saying, "Get out, O wicked soul, to fierce hot wind, boiling water and a shadow of black smoke."
The disbeliever's soul then scatters throughout his body, but the angels retrieve it, just as a needle is retrieved from wet wool. In this case, veins and nerve cells will be still attached to the soul.
Allah states here that angels bring news of the torment of the Fire to the disbelievers.
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