The 99 Names · The Closing Verses of Surat Al-Hashr
Al-Muhaymin المهيمن
The One who oversees hidden matters and the secrets of hearts; the Watcher and Witness over His servants and their deeds.
Explanation
A detailed explanation of this name (in Arabic) is in: Sharh Asma' Allah al-Husna fi Daw' al-Kitab wa al-Sunnah — Dr. Sa'eed ibn 'Ali ibn Wahf al-Qahtani — p. 172 ↗
Evidence
﴿هُوَ ٱللَّهُ ٱلَّذِي لَآ إِلَٰهَ إِلَّا هُوَ ٱلۡمَلِكُ ٱلۡقُدُّوسُ ٱلسَّلَٰمُ ٱلۡمُؤۡمِنُ ٱلۡمُهَيۡمِنُ ٱلۡعَزِيزُ ٱلۡجَبَّارُ ٱلۡمُتَكَبِّرُۚ سُبۡحَٰنَ ٱللَّهِ عَمَّا يُشۡرِكُونَ ٢٣﴾
Call upon Allah by this name
“To Allah belong the Beautiful Names, so call upon Him by them” — Al-A'raf 180
Example for practice
اللهم يا مهيمن، أنت المطّلع على سري وعلانيتي، فأصلح ما خفي من أمري كما أصلحت ظاهره
O Allah, O Al-Muhaymin, You see my secret and my open; set right what is hidden in my affairs as You set right what is visible.
An example for practice, not a transmitted text — following the Quranic way of closing a request with the fitting name (see Ibn al-Qayyim, Bada'i' al-Fawa'id 2/249).