It was narrated from ‘Abd-Allaah ibn ‘Amr ibn al-‘Aas (may Allaah be pleased with him) that the Prophet (peace and blessings of Allaah be upon him) said:“The best of du’a is du’a on the day of ‘Arafah, and the best that I and the Prophets before me said is:
Laa ilaaha illallaahu wahdahu laa shareeka lah, lahul-mulku walahul-hamdu wahuwa ‘ala kulli shay-in qadeer
[Narrated by al-Tirmidhi (3585) classed as hasan by al-Albaani in Saheeh al-Targheeb (1536)]