Allah reminds the believers of the slumber that He sent down on them as security from the fear they suffered from, because of the multitude of their enemy and the sparseness of their forces. They were given the same favor during the battle of Uhud, which Allah described,

﴿ثُمَّ أَنزَلَ عَلَيْكُمْ مِّن بَعْدِ الْغَمِّ أَمَنَةً نُّعَاساً يَغْشَى طَآئِفَةً مِّنْكُمْ وَطَآئِفَةٌ قَدْ أَهَمَّتْهُمْ أَنْفُسُهُمْ﴾

(Then after the distress, He sent down security for you. Slumber overtook a party of you, while another party was thinking about themselves. ) ﴿3:154﴾

Abu Talhah said, "I was among those who were overcome by slumber during (the battle of) Uhud. The sword fell from my hand several times, and I kept picking it up again, several times. I also saw the Companions' heads nodding while in the rear guard.'' Al-Hafiz Abu Ya`la narrated that `Ali said, "Only Al-Miqdad had a horse during Badr, and at some point, I found that all of us fell asleep, except the Messenger of Allah . He was praying under a tree and crying until dawn.'' `Abdullah bin Mas`ud said, "Slumber during battle is security from Allah, but during prayer, it is from Shaytan.'' Qatadah said, "Slumber affects the head, while sleep affects the heart.''

Slumber overcame the believers on the day of Uhud, and this incident is very well-known. As for this Ayah (8:11), it is describing the battle of Badr, indicating that slumber also overcame the believers during Badr. Therefore, it appears that this will occur for the believers, whenever they are in distress, so that their hearts feel safe and sure of Allah's aid, rewards, favor and mercy from Allah with them. Allah said in another Ayah,

﴿فَإِنَّ مَعَ الْعُسْرِ يُسْراً - إِنَّ مَعَ الْعُسْرِ يُسْراً ﴾

(Verily, along with every hardship is relief. Verily, along with every hardship is relief.) ﴿94:5-6﴾

In the Sahih, it is recorded that on the day of Badr, while he was in the bunker with Abu Bakr, the Messenger and Abu Bakr were invoking Allah. Suddenly, slumber overcame the Messenger and he woke up smiling and declared,

«أَبْشِرْ يَا أَبَابَكْرٍ هَذَا جِبْرِيلُ عَلَى ثَنَايَاهُ النَّقْع»

("Good news, O Abu Bakr! This is Jibril with dust on his shoulders.''

He left the shade while reciting Allah's statement,

﴿سَيُهْزَمُ الْجَمْعُ وَيُوَلُّونَ الدُّبُرَ ﴾

(Their multitude will be put to flight, and they will show their backs.) ﴿54:45﴾