There Is No God but Allah Alone
لَا إِلَٰهَ إِلَّا اللَّهُ وَحْدَهُ لَا شَرِيكَ لَهُ، لَهُ الْمُلْكُ وَلَهُ الْحَمْدُ وَهُوَ عَلَىٰ كُلِّ شَيْءٍ قَدِيرٌ
La ilaha illallahu wahdahu la sharika lah, lahul-mulku wa lahul-hamdu wa huwa 'ala kulli shay'in qadir.
There is no god but Allah alone, with no partner. To Him belongs the kingdom, to Him belongs all praise, and He is able to do all things.
What it means
The core statement of Islam with its consequences spelled out: if He alone is God, then ownership is His, praise is His, and power is His. Repeating it is a way of keeping the most important sentence you know from going stale.
When and how
Many recite it ten times after Fajr and ten times after Maghrib, and the full hundred is best spread through the day.
Its virtue
The Prophet ﷺ said that whoever says it a hundred times in a day has the reward of freeing ten slaves, a hundred good deeds are written for him, a hundred sins are erased, and he is protected from Shaytan that day until evening, reported in Sahih al-Bukhari 6403 and Sahih Muslim 2691.
Source
Al-Bukhari 6403; Muslim 2691sahih