اللهُ لاَ إِلَهَ إِلاَّ هُوَ الْحَيُّ الْقَيُّومُ لاَ تَأْخُذُهُ سِنَةٌ وَلاَ نَوْمٌ لَّهُ مَا فِي السَّمَاوَاتِ وَمَا فِي الأَرْضِ مَن ذَا الَّذِي يَشْفَعُ عِنْدَهُ إِلاَّ بِإِذْنِهِ يَعْلَمُ مَا بَيْنَ أَيْدِيهِمْ وَمَا خَلْفَهُمْ وَلاَ يُحِيطُونَ بِشَيْءٍ مِّنْ عِلْمِهِ إِلاَّ بِمَا شَاء وَسِعَ كُرْسِيُّهُ السَّمَاوَاتِ وَالأَرْضَ وَلاَ يَؤُودُهُ حِفْظُهُمَا وَهُوَ الْعَلِيُّ الْعَظِيمُ
Allahu la ilaha illa huwal-hayyul-qayyoom, la ta/khuthuhu sinatun wala nawm, lahu ma fis-samawati wama fil-ard, man thal-lathee yashfaAAu AAindahu illa bi-ithnih, yaAAlamu ma bayna aydeehim wama khalfahum, wala yuheetoona bishay-in min AAilmihi illa bima sha/, wasiAAa kursiyyuhus-samawati wal-ard, wala yaooduhu hifthuhumaL wahuwal-AAaliyyul-AAatheem
Translation
Allah — there is no deity except Him, the Ever-Living, the Sustainer of existence. Neither drowsiness overtakes Him nor sleep. To Him belongs whatever is in the heavens and whatever is on the earth. Who is it that can intercede with Him except by His permission? He knows what is before them and what will be after them, and they encompass not a thing of His knowledge except for what He wills. His Throne extends over the heavens and the earth, and their preservation tires Him not. And He is the Most High, the Most Great.
When to recite
Recited once after every obligatory prayer. Whoever recites it will be in the protection of Allah until the next prayer.
Source / Reference
An-Nasa'i, Al-Hakim (Sahih)