Abu Qatadah (may Allah be pleased with him) narrates from the Prophet (peace and blessings be upon him) that he (peace and blessings be upon him) said: "Fasting on the Day of Arafah is an expiation for two years, one coming year and one past year, and fasting on the day of Ashura is an expiation for one year."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8383