It was narrated that ‘Aa’ishah (may Allaah be pleased with her) said: The Messenger of Allaah (peace and blessings of Allaah be upon him) said:“There is no day on which Allaah ransoms more of His slaves from the Fire than the day of ‘Arafah. He draws close then He boasts about them before the angels and says, ‘What do these people want?’” Narrated by Muslim (1348).
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 ill-Allaah wahdahu la shareeka lah, lahu’l-mulk wa lahu’l-hamd wa huwa ‘ala kulli shay’in qadeer”
(There is no god but Allaah alone, with no partner or associate; His is the dominion, to Him be praise, and He has power over all things)
Narrated by al-Tirmidhi