The Prophet ﷺ was asked about fasting on the Day of Arafah. He replied, "It expiates the sins of the past year and the coming year."
وسئل (يعني النبي ﷺ) عن صوم يوم عرفة؟ فقال: يكفر السنة الماضية والباقية.
Muslim narrated it in the Book of Fasting (1162: 197) through Shuba, from Ghaylan bin Jareer, who heard Abdullah bin Mabad Zimmani, from Abu Qatada, and he mentioned it. This is part of a long hadith that has already been mentioned in full.Muslim also narrated it (1162: 196) through Hammad bin Zayd, from Ghaylan bin Jareer, with the same wording, and in it, "As for fasting the day of Arafah, I reckon that Allah will forgive through it the sins of the year before it and the year after it."