النبي (صلى الله عليه وسلم) said: 'The best الدعاء is the الدعاء of the Day of عرفة, and the best thing that I and the prophets before me said is: La ilaha illallahu wahdahu la shareeka lah, lahul-mulku wa lahul-hamdu wa huwa ala kulli shay'in qadeer.' This declaration of tawheed is the greatest dhikr for the greatest day of the year.
لَا إِلَٰهَ إِلَّا اللَّهُ وَحْدَهُ لَا شَرِيكَ لَهُ، لَهُ الْمُلْكُ وَلَهُ الْحَمْدُ وَهُوَ عَلَىٰ كُلِّ شَيْءٍ قَدِيرٌ
La ilaha illallahu wahdahu la shareeka lah, lahul-mulku wa lahul-hamdu wa huwa 'ala kulli shay'in qadeer.
Meaning: There is no god but Allah alone, with no partner. His is the dominion and His is the praise, and He is over all things capable.
Source: Jami' al-Tirmidhi #0The Day of عرفة is the greatest day of the entire year. النبي (صلى الله عليه وسلم) said: 'There is no day on which Allah frees more people from the Fire than the Day of عرفة. He draws near and then boasts about them to the angels, saying: What do these people want?' (صحيح مسلم 1348). In another narration, Allah says to the angels: 'Bear witness that I have forgiven them' (reported by Ibn Hibban, graded sahih by الألباني). النبي (صلى الله عليه وسلم) also said: 'Fasting the Day of عرفة expiates the sins of the previous year and the coming year' (صحيح مسلم 1162) — this applies to non-الحجاج. For الحجاج at عرفة, it is the moment when sins are forgiven, الأدعية are answered, and slaves are freed from the Hellfire in the greatest numbers. Shaykh ابن باز said that the الحاج should not waste a single moment of this day and should persistently call upon Allah with sincerity and hope.
لَا إِلَٰهَ إِلَّا اللَّهُ وَحْدَهُ لَا شَرِيكَ لَهُ، لَهُ الْمُلْكُ وَلَهُ الْحَمْدُ وَهُوَ عَلَىٰ كُلِّ شَيْءٍ قَدِيرٌ
There is no god but Allah alone, with no partner. His is the dominion and His is the praise, and He is over all things capable.