Sayyiduna Abu Qatadah (may Allah be pleased with him) narrates that this reached him from the Prophet (peace and blessings be upon him): "Fasting on the Day of Arafah is an expiation for one year and for the one that follows it, and fasting on the day of Ashura is an expiation for one year."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8381