Jabir reported that the Prophet (peace and blessings be upon him) said, “There is no day better in the sight of Allah than the Day of Arafah. On this day Allah, the Almighty and the Exalted One, descends to the nearest heaven, and He is proud of His slaves on the earth and says to those in heaven, ‘Look at My servants. They have come from far and near, with hair dishevelled and faces covered with dust, to seek My mercy, even though they have not seen My chastisement.' Far more people are freed from the Hellfire on the Day of Arafah than on any other day.” [Ibn Hibban]
The Prophet (peace and blessings be upon him) spent the day at Arafah until almost sunset. Then he said, “O Bilal, ask the people to be quiet and listen to me.” Bilal stood up and asked the people to be quiet and listen to the Prophet (peace and blessings be upon him). When the people were quiet, the Prophet (peace and blessings be upon him) said, “O people, a little while ago Gabriel (peace be upon him) came to me. Gave me salutations from Allah and informed me that Allah has forgiven those who spend the Day at Arafah and those who stop at Al-Mashar Al-Haram, and that He has guaranteed their debts.”
At this Umar ibn Al-Khattab stood up and asked, “O Allah’s Messenger, is this for us only?” The Prophet (peace and blessings be upon him) said, “This is for you and for all those who will come after you until the Day of Judgment.” Umar exclaimed, “How plentiful and blissful Allah’s bounties are!” [Ibn Al-Mubarak]
Abu Ad-Darda reported that the Prophet (peace and blessings be upon him) said, “On no other day does the Satan feel so belittled, humiliated, and angry as he does on the Day of Arafah. The reason for this is the mercy of Allah that descends (this day) and the forgiveness that He grants to people for major sins, except the day of the Battle of Badr, which witnessed a far greater mercy of Allah descending upon people, which caused great sadness to Satan."
The Prophet (peace and blessings be upon him) was asked, “O Prophet of Allah, what did Satan see on the day of the Battle of Badr?” “He saw Gabriel leading the troops of angels,” he replied (Reported by Malik and Al-Hakim).
"Fiqh-us-Sunnah" - Sayyid Sabiq