Nabi (shallallahu alaihi wa sallam) said: 'The best doa is the doa of the Day of Arafah, 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 Arafah is the greatest day of the entire year. Nabi (shallallahu alaihi wa sallam) said: 'There is no day on which Allah frees more people from the Fire than the Day of Arafah. 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). Nabi (shallallahu alaihi wa sallam) also said: 'Fasting the Day of Arafah expiates the sins of the previous year and the coming year' (Sahih Muslim 1162) — this applies to non-jamaah haji. For jamaah haji at Arafah, it is the moment when sins are forgiven, doa-doa are answered, and slaves are freed from the Hellfire in the greatest numbers. Shaykh Ibn Baz said that the jamaah haji 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.