Narrated by Amr Ibn Shuaib (RA) from his grandfather through his father, the Prophet (ﷺ) said, “The best dua is the dua on the day of Arafa. The best prayer I and my predecessors have read is -

لَا إِلَهَ إِلَّا اللَّهُ وَحْدَهُ لَا شَرِيكَ لَهُ، لَهُ الْمُلْكُ وَلَهُ الْحَمْدُ وَهُوَ عَلَى كُلِّ شَيْءٍ قَدِيرٌ

Laa 'ilaaha 'illAllahu wahdahu laa shareeka lahu, lahul-mulku wa lahul-hamdu wa Huwa 'alaa kulli shay'in Qadeer

None has the right to be worshipped except Allah, alone, without any partener. To Him belong all sovereignty and praise and He is over all things omnipotent. Narrated by Amr Ibn Shuaib (RA) from his grandfather through his father, the Prophet (ﷺ) said, “The best dua is the dua on the day of Arafa. The best prayer I and my predecessors have read is - (The dua is Mentioned above)

Reference: The hadith is Hasan. Tirmidhi, 3585