Surah Ali 'Imran 3:154

ثُمَّ أَنزَلَ عَلَيْكُم مِّنۢ بَعْدِ ٱلْغَمِّ أَمَنَةً نُّعَاسًا يَغْشَىٰ طَآئِفَةً مِّنكُمْ ۖ وَطَآئِفَةٌ قَدْ أَهَمَّتْهُمْ أَنفُسُهُمْ يَظُنُّونَ بِٱللَّهِ غَيْرَ ٱلْحَقِّ ظَنَّ ٱلْجَـٰهِلِيَّةِ ۖ يَقُولُونَ هَل لَّنَا مِنَ ٱلْأَمْرِ مِن شَىْءٍ ۗ قُلْ إِنَّ ٱلْأَمْرَ كُلَّهُۥ لِلَّهِ ۗ يُخْفُونَ فِىٓ أَنفُسِهِم مَّا لَا يُبْدُونَ لَكَ ۖ يَقُولُونَ لَوْ كَانَ لَنَا مِنَ ٱلْأَمْرِ شَىْءٌ مَّا قُتِلْنَا هَـٰهُنَا ۗ قُل لَّوْ كُنتُمْ فِى بُيُوتِكُمْ لَبَرَزَ ٱلَّذِينَ كُتِبَ عَلَيْهِمُ ٱلْقَتْلُ إِلَىٰ مَضَاجِعِهِمْ ۖ وَلِيَبْتَلِىَ ٱللَّهُ مَا فِى صُدُورِكُمْ وَلِيُمَحِّصَ مَا فِى قُلُوبِكُمْ ۗ وَٱللَّهُ عَلِيمٌۢ بِذَاتِ ٱلصُّدُورِ

Then after distress, He sent down upon you security [in the form of] drowsiness, overcoming a faction of you, while another faction worried about themselves, thinking of Allāh other than the truth - the thought of ignorance, saying, "Is there anything for us [to have done] in this matter?" Say, "Indeed, the matter belongs completely to Allāh." They conceal within themselves what they will not reveal to you. They say, "If there was anything we could have done in the matter, we [i.e., some of us] would not have been killed right here." Say, "Even if you had been inside your houses, those decreed to be killed would have come out to their death beds." [It was] so that Allāh might test what is in your breasts and purify what is in your hearts. And Allāh is Knowing of that within the breasts.

Translation: Saheeh International

Tafsir

Then He sent down upon you after grief security — a slumber nu‘āsan ‘slumber’ substitutes for amanatan ‘security’ overcoming yaghshā or taghshā a party of you namely the believers. They would become dizzy under their shields and their swords would fall from their hands; …

Excerpt from Tafsir al-Jalalayn, commentary on 3:154, via Quran.com. The reader has the full commentary and 9 more tafsirs.