The Messenger of Allah ﷺ was asked about fasting on the day of Arafah, and he said, "It is an expiation for two years."He was asked about fasting on the day of Ashura, and he said, "It is an expiation for one year."
حدثنا هشيم بن بشير أخبرنا منصور يعني ابن زاذان عن قتادة عن عبد الله بن معبد الزماني
أن رسول الله ﷺ سئل عن صوم يوم عرفة فقال كفارة سنتين وسئل عن صوم يوم عاشوراء فقال كفارة سنة
Its isnad is sahih. Abdullah bin Mabad Zimmani is a well-known trustworthy narrator, and his hadith appears in Muslim. Muslim narrated the hadith (2/818, no. 1162), and Ibn Majah (1/551, no. 1730), both in the book of fasting.