(فَرِحَ الْمُخَلَّفُونَ…) (Those who remained behind rejoiced): Joy is the delight emotion because of an action that brings happiness to the soul. Those who remained behind due to their hypocrisy and chose to remain in al-Medina, abandoning the Prophet (saws) during the battle of Tabuk, which took place in the intense time of summer. They presented false excuses, and the Prophet (saws) left them behind because Allah (swt) revealed:

  لَوْ خَرَجُوا فِيكُم مَّا زَادُوكُمْ إِلَّا خَبَالًا وَلَأَوْضَعُوا خِلَالَكُمْ يَبْغُونَكُمُ الْفِتْنَةَ وَفِيكُمْ سَمَّاعُونَ لَهُمْ ۗ وَاللَّهُ عَلِيمٌ بِالظَّالِمِينَ 

(Had they gone forth with you, they would not have increased you except in confusion, and they would have been active among you, seeking [to cause] you fitnah. And among you are avid listeners to them. And Allah is Knowing of the wrongdoers) (At-Tawbah: 47)

 

…بِمَقْعَدِهِمْ…  (in their staying [at home]): The Arabic word of (their staying) indicates the place of sitting, so this reflects immobility, as it is remaining in a particular place, while standing is a symbol of activeness, initiating movement from one place to another.

…خِلَافَ رَسُولِ اللَّهِ…  (after [the departure of] the Messenger of Allah,): Meaning, they disagreed the opinion of the Prophet (saws) and have not departed with him. Allah (swt) explains the reason behind their staying back saying:

…وَكَرِهُوا أَن يُجَاهِدُوا بِأَمْوَالِهِمْ وَأَنفُسِهِمْ فِي سَبِيلِ اللَّهِ…  (and (they) disliked to strive with their wealth and their lives in the cause of Allah,): Not only that, they also intended to demoralize the spirits of the believers and compel them to stay in al-Medina. They said to them:

…وَقَالُوا لَا تَنفِرُوا فِي الْحَرِّ ۗ…  (…and said, ‘Do not go forth in the heat,’): They were not content with their own disgraceful attitude; in fact, they started inciting the believers not to fight and suggested an excuse for them too, claiming that the weather was hot and there would be hardship.

…قُلْ…  (say,): Oh Muhammad.

…نَارُ جَهَنَّمَ أَشَدُّ حَرًّا ۚ…  (The fire of Hell is more intensive in heat,): If they had fled from heat and hardship, then the hardship of the fire of Hell is much greater. Man cannot comprehend the fire of Hell. Thus, Allah (swt) has informed them that this matter will be their ultimate destination.

…لَّوْ كَانُوا يَفْقَهُونَ  (if they would but understand.): The Arabic word ( understand) means an accurate and in-depth understanding of something. If you grasp something superficially, you are said to ‘know’ it. But if you comprehend it with all its data and backgrounds, understanding its reality, then you have truly understood it. It is true that they would have faced hardship by going to the Battle of Tabuk in the heat, but they do not understand that by staying behind, the punishment would be greater and more severe.