The Prophet ﷺ was asked about fasting on the day of Ashura. 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) via Ghaylan bin Jareer, who heard Abdullah bin Mabad Zimmani, from Abu Qatada, who mentioned it. This is part of a longer hadith, which has already been mentioned in full.