Allah reminds His servants of His favor when He sent down on them tranquillity and slumber that overcame them while they were carrying their weapons and feeling distress and grief. In this case, slumber is a favor and carries meanings of calmness and safety. For instance, Allah said in Surat Al-Anfal about the battle of Badr,
إِذْ يُغَشِّيكُمُ النُّعَاسَ أَمَنَةً مِّنْهُ
((Remember) when He covered you with a slumber as a security from Him) 8:11.
Al-Bukhari recorded that Anas said that, Abu Talhah said, "I was among those who were overcome by slumber during the battle of Uhud. My sword fell from my hand several times and I would pick it up, then it would fall and I would pick it up again." Al-Bukhari collected this Hadith in the stories of the battles without a chain of narration, and in the book of Tafsir with a chain of narrators. At-Tirmidhi, An-Nasa'i and Al-Hakim recorded from Anas that Abu Talhah said, "On the day of Uhud, I raised my head and looked around and found that everyone's head was nodding from slumber." This is the wording of At-Tirmidhi, who said, "Hasan Sahih". An-Nasa'i also recorded this Hadith from Anas who said that Abu Talhah said, "I was among those who were overcome by slumber."
The second group mentioned in the Ayah were the hypocrites who only thought about themselves, for they are the most cowardly people and those least likely to support the truth,
يَظُنُّونَ بِاللَّهِ غَيْرَ الْحَقِّ ظَنَّ الْجَـهِلِيَّةِ
(and thought wrongly of Allah - the thought of ignorance) 3:154, for they are liars and people who have doubts and evil thoughts about Allah, the Exalted and Most Honored. Allah said,
ثُمَّ أَنزَلَ عَلَيْكُمْ مِّن بَعْدِ الْغَمِّ أَمَنَةً نُّعَاساً يَغْشَى طَآئِفَةً مِّنْكُمْ
(Then after the distress, He sent down security for you. Slumber overtook a party of you), the people of faith, certainty, firmness and reliance (on Allah) who are certain that Allah shall give victory to His Messenger and fulfill his objective.
وَطَآئِفَةٌ قَدْ أَهَمَّتْهُمْ أَنْفُسُهُمْ
(While another party was thinking about themselves), and they were not overcome by slumber because of their worry, fright and fear,
يَظُنُّونَ بِاللَّهِ غَيْرَ الْحَقِّ ظَنَّ الْجَـهِلِيَّةِ
(and thought wrongly of Allah --- the thought of ignorance).
Similarly, Allah said in another statement,
بَلْ ظَنَنْتُمْ أَن لَّن يَنقَلِبَ الرَّسُولُ وَالْمُؤْمِنُونَ إِلَى أَهْلِيهِمْ أَبَداً
(Nay, but you thought that the Messenger and the believers would never return to their families) 48:12.
This group thought that the idolators achieved ultimate victory, when their forces took the upper hand in battle, and that Islam and its people would perish. This is typical of people of doubt and hesitation, in the event of a hardship, they fall into such evil thoughts. Allah then described them that,
يَقُولُونَ
(they said) in this situation,
هَل لَّنَا مِنَ الاٌّمْرِ مِن شَىْءٍ
("Have we any part in the affair") Allah replied,
قُلْ إِنَّ الاٌّمْرَ كُلَّهُ للَّهِ يُخْفُونَ فِى أَنْفُسِهِم مَّا لاَ يُبْدُونَ لَكَ
(Say: "Indeed the affair belongs wholly to Allah." They hide within themselves what they dare not reveal to you.) wAllah exposed their secrets, that is,
يَقُولُونَ لَوْ كَانَ لَنَا مِنَ الاٌّمْرِ شَىْءٌ مَّا قُتِلْنَا هَـهُنَا
(saying: "If we had anything to do with the affair, none of us would have been killed here.") although they tried to conceal this thought from the Messenger of Allah ﷺ.
Ibn Ishaq recorded that `Abdullah bin Az-Zubayr said that Az-Zubayr said, "I was with the Messenger of Allah ﷺ when fear intensified and Allah sent sleep to us (during the battle of Uhud). At that time, every man among us (except the hypocrites) was nodding off. By Allah! As if in a dream, I heard the words of Mu`attib bin Qushayr, `If we had anything to do with the affair, none of us would have been killed here.' I memorized these words of his, which Allah mentioned later on,
يَقُولُونَ لَوْ كَانَ لَنَا مِنَ الاٌّمْرِ شَىْءٌ مَّا قُتِلْنَا هَـهُنَا
(saying: "If we had anything to do with the affair, none of us would have been killed here.)"
Ibn Abi Hatim collected this Hadith.
Allah the Exalted said,
قُل لَّوْ كُنتُمْ فِى بُيُوتِكُمْ لَبَرَزَ الَّذِينَ كُتِبَ عَلَيْهِمُ الْقَتْلُ إِلَى مَضَاجِعِهِمْ
(Say: "Even if you had remained in your homes, those for whom death was decreed would certainly have gone forth to the place of their death,") meaning, this is Allah's appointed destiny and a decision that will certainly come to pass, and there is no escaping it. Allah's statement,
وَلِيَبْتَلِىَ اللَّهُ مَا فِى صُدُورِكُمْ وَلِيُمَحِّصَ مَا فِى قُلُوبِكُمْ
(that Allah might test what is in your breasts; and to purify that which was in your hearts,) means, so that He tests you with whatever befell you, to distinguish good from evil and the deeds and statements of the believers from those of the hypocrites,
وَاللَّهُ عَلِيمٌ بِذَاتِ الصُّدُورِ
(and Allah is All-Knower of what is in the breasts), and what the hearts conceal.
Allah then said,
إِنَّ الَّذِينَ تَوَلَّوْاْ مِنكُمْ يَوْمَ الْتَقَى الْجَمْعَانِ إِنَّمَا اسْتَزَلَّهُمُ الشَّيْطَـنُ بِبَعْضِ مَا كَسَبُواْ
(Those of you who turned back on the day the two hosts met, Shaytan only caused them to err because of some of what they had earned) 3:155,
because of some of their previous errors. Indeed, some of the Salaf said, "The reward of the good deed includes being directed to another good deed that follows it, while the retribution of sin includes committing another sin that follows it." Allah then said,
وَلَقَدْ عَفَا اللَّهُ عَنْهُمْ
(but Allah, indeed, has forgiven them), their giving flight,
أَنَّ اللَّهَ غَفُورٌ حَلِيمٌ
(surely, Allah is Oft-Forgiving, Most Forbearing)
He forgives sins, pardons and exonerates His creatures. Imam Ahmad recorded that Shaqiq said, " `Abdur-Rahman bin `Awf met Al-Walid bin `Uqbah, who said to him, `Why did you desert `Uthman, the Leader of the Faithful' `Abdur-Rahman said, `Tell him that I did not run away during Uhud, remain behind during Badr, nor abandon the Sunnah of `Umar.' Al-Walid told `Uthman what `Abdur-Rahman said. `Uthman replied, `As for his statement, `I did not run away during Uhud,' how can he blame me for an error that Allah has already forgiven. Allah said,
إِنَّ الَّذِينَ تَوَلَّوْاْ مِنكُمْ يَوْمَ الْتَقَى الْجَمْعَانِ إِنَّمَا اسْتَزَلَّهُمُ الشَّيْطَـنُ بِبَعْضِ مَا كَسَبُواْ وَلَقَدْ عَفَا اللَّهُ عَنْهُمْ
(Those of you who turned back on the day the two hosts met, Shaytan only caused them to err because of some of what they had earned. But Allah, indeed, has forgiven them).
As for his statement that I remained behind from participating in Badr, I was nursing Ruqayyah, the daughter of the Messenger of Allah ﷺ, until she passed away. The Messenger of Allah ﷺ gave me a share in the booty of Badr, and whoever gets a share in the booty from the Messenger of Allah ﷺ will have participated in battle. As for his statement that I abandoned the Sunnah of `Umar, neither I nor he are able to endure it. Go and convey this answer to him."'