The Messenger of Allah ﷺ said, "Whoever fasts on the Day of Arafah will have two consecutive years of sins forgiven."
قال رسول الله ﷺ: من صام يوم عرفة، غفر له سنتين متتابعتين.
Abu Yaala (7548) narrated it from Abu Bakr bin Abu Shayba, who said, "Muawiya bin Hisham told us, from Abu Hafs Taifi, from Abu Hazim, from Sahl bin Saad," and he related it. It is also found in the Musannaf of Abu Bakr bin Abu Shayba (3/97), and from this route Tabarani also narrated it in al-Kabir (6/220). Its isnad is hasan because of Abu Hafs, who is Abdul-Salam bin Hafs, Abu Hafs, also said to be Abu Musab Madani, also said to be Taifi, also said to be Qurashi, their freedman. Ibn Main deemed him reliable. Abu Hatim said, "He is not well known." Ibn Hibban mentioned him in "al-Thiqat." Dhahabi said in "al-Diwan," "Truthful, but he produces unusual reports." The isnad also contains Muawiya bin Hisham, known as al-Qassar, one of the men of the Sahih, except that he has been described as having lapses.As for Haythami's statement in "al-Majma" (1893), "Abu Yaala and Tabarani narrated it in al-Kabir, and the men of Abu Yaala's narration are the men of the Sahih," it contains two errors. The first is that Abdul-Salam bin Hafs is not among the men of the Sahih, rather he was narrated from by the compilers of the Sunan works other than Ibn Majah. The second is that his wording gives the impression that Tabarani narrated it through another route, whereas the correct position is that he too narrated it through the route of Ibn Abu Shayba, just as Abu Yaala did, except that he added another route, namely Uthman bin Abu Shayba, both of them narrating from Muawiya bin Hisham.On this subject there is also what was narrated from Qatada bin Numan, who said, "I heard the Messenger of Allah ﷺ say, 'Whoever fasts the day of Arafah, a year before it and a year after it will be forgiven for him.'" Ibn Majah (1731) narrated it from Hisham bin Ammar, who said, "Yahya bin Hamza told us, from Ishaq bin Abdullah, from Iyad bin Abdullah, from Abu Saeed Khudri, from Qatada bin Numan," and he related it. Its isnad is very daif, for it contains Ishaq bin Abdullah, who is Ibn Abu Farwa, daif by the agreement of the scholars. Abu Hatim, Nasai, Daraqutni, and others said, "His hadith is abandoned."On this subject there is also what was narrated from Ayesha, who said, "I heard the Messenger of Allah ﷺ say, 'The fast of Arafah atones for the year that precedes it.'" Imam Ahmad (24970) narrated it from Affan, who said, "Hammad bin Salama told us, who said, Ata Khurasani informed us, that Abdul-Rahman bin Abu Bakr came in to see Ayesha on the day of Arafah while she was fasting, and water was being sprinkled on her. Abdul-Rahman said to her, 'Break your fast.' She said, 'Should I break it? I heard the Messenger of Allah ﷺ say,'" and she related it. Its isnad is disconnected, for Ata Khurasani did not hear from Ayesha. Munziri faulted it on this basis in "al-Targheeb wal-Tarheeb" (1538), as did Haythami in "al-Majma" (3/189).His statement, "the day of Arafah," contains a reference to the place called Arafah, where the pilgrims gather to perform the rites of Hajj. The scholars differ over defining the day of Arafah, holding two views. Some scholars hold that what is meant by the day of Arafah is the ninth day of Dhul-Hijjah according to each country's own sighting, and they have evidence for this. Others hold that what is meant is the day on which the pilgrims gather in this place. In the fatwas of the Permanent Committee (no. 104052) it states, "The day of Arafah is the day on which people stand at Arafah, and fasting it is prescribed for anyone not engaged in the Hajj. So if you wish to fast, you should fast this day."This is what I consider closest to being correct, because the hadiths conveying the merit of fasting this day came with the wording "the day of Arafah," not with the wording "the ninth day of Dhul-Hijjah" according to each country's sighting. If "the ninth day" appears in some routes, it is an explanation from the narrator of the day of Arafah. However, a problem may arise in that in some country the day of Arafah could be the tenth day of Dhul-Hijjah according to the sighting, and fasting the tenth day, which is the day of Eid al-Adha, is forbidden. For such people it is permissible to fast the ninth day according to their country's sighting, unlike those for whom the day of Arafah is the eighth day of Dhul-Hijjah.Many hadiths have come regarding the general merit of the day of Arafah, which Bayhaqi strengthened through their supporting narrations, though they do not meet the standard of Jami Kamil. Still, they may be drawn on for support regarding the merit of the day of Arafah, not specifically the ninth day.