Abu Qatadah (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) was asked about fasting on the day of Ashura, and he (peace and blessings be upon him) said: "It is an expiation for the sins of one year." And he (peace and blessings be upon him) was asked about fasting on the Day of Arafah, and he (peace and blessings be upon him) said: "It is an expiation for two years: one past year and one coming year."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8382