وَأَنذِرْهُمْ يَوْمَ الْحَسْرَةِ ...

And warn them of the Day of grief and regrets,

warn the creation of the Day of Distress,

... إِذْ قُضِيَ الْأَمْرُ ...

when the case has been decided,

when the people of Paradise and the people of Hell will be sorted out, and everyone will reach his final abode which he was destined to remain in forever.

... وَهُمْ ...

while (now) they are,

today, in the present life of this world,

... فِي غَفْلَةٍ ...

in a state of carelessness.

with the warning of the Day of grief and regret, they are heedless.

... وَهُمْ لَا يُؤْمِنُونَ ﴿٣٩﴾

and they believe not.

meaning they do not believe that it is true.

Imam Ahmad recorded that Abu Sa`id said that the Messenger of Allah said,

إِذَا دَخَلَ أَهْلُ الْجَنَّةِ الْجَنَّةَ وَأَهْلُ النَّارِ النَّارَ، يُجَاءُ بِالْمَوْتِ كَأَنَّهُ كَبْشٌ أَمْلَحُ فَيُوقَفُ بَيْنَ الْجَنَّةِ وَالنَّارِ، فَيُقَالُ: يَا أَهْلَ الْجَنَّةِ هَلْ تَعْرِفُونَ هَذَا،

قَالَ: فَيَشْرَئِبُّونَ وَيَنْظُرُونَ وَيَقُولُونَ:نَعَمْ هَذَا الْمَوْتُ

قَالَ: فَيُقَالُ: يَاأَهْلَ النَّارِ، هَلْ تَعْرِفُونَ هَذَا؟

قَالَ: فَيَشْرَئِبُّونَ وَيَنْظُرُونَ وَيَقُولُونَ: نَعَمْ هَذَا الْمَوْتُ

قَالَ: فَيُؤْمَرُ بِهِ فَيُذْبَحُ، قَالَ: وَيُقَالُ: يَا أَهْلَ الْجَنَّةِ، خُلُودٌ وَلَا مَوْتَ، وَيَا أَهْلَ النَّارِ، خُلُودٌ وَلَا مَوْت

When the people of Paradise enter Paradise and the people of the Hellfire enter the Hellfire, death will be brought in the form of a handsome ram and it will be placed between Paradise and the Hellfire. Then, it will be said, "O people of Paradise, do you know what this is?''

Then, they will turn their gazes and look, and they will say, "Yes, this is death.''

Then, it will be said, "O people of the Hellfire, do you know what this is?''

Then, they will turn their gazes and look, and they will say, "Yes, this is death.''

Then, the order will be given for it to be slaughtered and it will be said, "O people of Paradise, eternity and no more death, O people of Hellfire, eternity and no more death.''

Then the Messenger of Allah recited the Ayah,

وَأَنذِرْهُمْ يَوْمَ الْحَسْرَةِ إِذْ قُضِيَ الْأَمْرُ وَهُمْ فِي غَفْلَةٍ وَهُمْ لَا يُؤْمِنُونَ

And warn them of the Day of grief and regret, when the case has been decided, while (now) they are in a state of carelessness, and they believe not.

Then, the Messenger of Allah made a gesture with his hand and said,

أَهْلُ الدُّنْيَا فِي غَفْلَةِ الدُّنْيَا

The people of this life are in the state of heedlessness of this life.

Thus recorded Imam Ahmad and it was also recorded by Al-Bukhari and Muslim in their Sahihs with wording similar to this.

It is reported from Abdullah bin Mas`ud that he mentioned a story in which he said,

"There is not a soul except that it will see a residence in Paradise and a residence in the Hellfire, and this will be the Day of distress.

So the people of the Hellfire will see the residence that Allah prepared for them if they had believed. Then, it will be said to them, `If you had believed and worked righteous deeds, you would have had this, which you see in Paradise.' Then, they will be overcome with distress and grief.

Likewise, the people of Paradise will see the residence that is in the Hellfire and it will be said to them, `If Allah had not bestowed His favor upon you (this would have been your place).'''