The Prophet (peace be upon him) said: 'The best supplication is the supplication of the Day of Arafat, 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 Arafat is the greatest day of the entire year. The Prophet (peace be upon him) said: 'There is no day on which Allah frees more people from the Fire than the Day of Arafat. He draws near and then boasts about them to the angels, saying: What do these people want?' (Sahih Muslim 1348). In another narration, Allah says to the angels: 'Bear witness that I have forgiven them' (reported by Ibn Hibban, graded sahih by al-Albani). The Prophet (peace be upon him) also said: 'Fasting the Day of Arafat expiates the sins of the previous year and the coming year' (Sahih Muslim 1162) — this applies to non-pilgrims. For pilgrims at Arafat, it is the moment when sins are forgiven, duas are answered, and slaves are freed from the Hellfire in the greatest numbers. Shaykh Ibn Baz said that the pilgrim 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.