Slumber

The triliteral root nūn ʿayn sīn (ن ع س) occurs in the Quran 2 times.

(3:154) Then after the worry, He sent down to you sanctity in slumber (Arab. نُعَاسًا, nuʿāsan), overtaking a group of you; while another group was worried about themselves; they were thinking about God other than the truth, the thoughts of the days of ignorance. They say, "Why are we involved in this affair?" Say, "The entire affair is up to God." They hide in their persons what they do not show to you; they say, "If we had a say in this affair then none of us would have been killed here." Say, "If you were inside your own homes, then the ones who have been marked for death would have gone forth to their resting place." God will test what is in your chests and bring out what is in your hearts. God is Knowledgeable as to what is in the chests.

(8:11) The peaceful slumber (Arab. النُّعَاسَ, l-nuʿāsa) from Him overcame you, and He sent down to you water from the sky to cleanse you with it. He caused the affliction of the devil to leave you so that He may fortify your hearts and set firm your feet.

Controversial issue

"Sinatun-Slumber سِنَةٌ" or "Sunnatu-Sunnah سُنَّةُ"

(2:255) God, there is no god but He, the Living, the Sustainer. No sunnah (Arab. سُنَّةُ, sunnatu) or sleep overtakes Him; to Him belongs all that is in the heavens and in the earth. Who will intercede with Him except by His leave? He knows their present and their future, and they do not have any of His knowledge except for what He wishes. His throne encompasses all of the heavens and the earth and it is easy for Him to preserve them. He is the High, the Great.