Ayat al-Kursi
اللَّهُ لَا إِلَٰهَ إِلَّا هُوَ الْحَيُّ الْقَيُّومُ لَا تَأْخُذُهُ سِنَةٌ وَلَا نَوْمٌ لَهُ مَا فِي السَّمَاوَاتِ وَمَا فِي الْأَرْضِ مَنْ ذَا الَّذِي يَشْفَعُ عِنْدَهُ إِلَّا بِإِذْنِهِ يَعْلَمُ مَا بَيْنَ أَيْدِيهِمْ وَمَا خَلْفَهُمْ وَلَا يُحِيطُونَ بِشَيْءٍ مِنْ عِلْمِهِ إِلَّا بِمَا شَاءَ وَسِعَ كُرْسِيُّهُ السَّمَاوَاتِ وَالْأَرْضَ وَلَا يَئُودُهُ حِفْظُهُمَا وَهُوَ الْعَلِيُّ الْعَظِيمُ
Allahu la ilaha illa huwal-hayyul-qayyum. La ta'khudhuhu sinatun wa la nawm. Lahu ma fis-samawati wa ma fil-ard. Man dhal-ladhi yashfa'u 'indahu illa bi-idhnih. Ya'lamu ma bayna aydihim wa ma khalfahum. Wa la yuhituna bi-shay'in min 'ilmihi illa bima sha'. Wasi'a kursiyyuhus-samawati wal-ard. Wa la ya'uduhu hifzuhuma wa huwal-'aliyyul-'azim.
Allah, there is no god but Him, the Ever-Living, the Sustainer of all that exists. Neither drowsiness nor sleep overtakes Him. To Him belongs whatever is in the heavens and whatever is on the earth. Who could intercede with Him except by His permission? He knows what lies before them and what lies behind them, and they grasp nothing of His knowledge except what He wills. His Throne extends over the heavens and the earth, and guarding them does not tire Him. He is the Most High, the Most Great.
What it means
This single verse is the clearest description in the Quran of who Allah is: alive without end, holding everything up, never tired, never caught off guard. Saying it is not a charm you wear; it is a statement you make about reality, and the calm it brings comes from meaning it. Everything you are anxious about is inside the domain this verse describes.
When and how
Recite it once in the morning, once in the evening, once before sleeping, and after each of the five prayers. Read it slowly enough to hear yourself.
Its virtue
The Prophet ﷺ taught that whoever recites it when going to bed has a protector from Allah over him and no devil comes near him until morning, as reported in Sahih al-Bukhari 2311. He also said that whoever recites it after every obligatory prayer has nothing between him and Paradise except death, reported in Sunan an-Nasa'i.
Source
Quran 2:255
Quranic text from Tanzil.net, Creative Commons Attribution 3.0.