Sayyiduna Abu Qatadah al-Ansari (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 Arafah, and he (peace and blessings be upon him) said: "It becomes an expiation for the sins of the past year and the coming year."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8380
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
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
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