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) via Ghaylan bin Jareer, who heard Abdullah bin Mabad Zimmani narrate from Abu Qatada, and he mentioned it. It is part of a longer hadith that has already been mentioned in full. In one wording, "Fasting the day of Arafah, I hope Allah will expiate for it the year before it and the year after it."