The Prophet ﷺ was asked about fasting on the day of Ashura, and he replied, "It expiates the sins of the previous year."
سئل النبي ﷺ عن صوم يوم عاشوراء؟ فقال: ((يكفر السنة الماضية)).
Muslim narrated it in the Book of Fasting (1162: 197) via Shubah, from Ghaylan bin Jareer, who heard Abdullah bin Mabad Zimmani, from Abu Qatada, and he mentioned it. It is part of a long hadith, which was already mentioned in full earlier.Muslim also narrated it (1162: 196) via Hammad bin Zayd, from Ghaylan bin Jareer, with the same account, and in it, "As for fasting the day of Ashura, I hope that Allah will use it to atone for the year before it."