Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) narrated: The Prophet (peace and blessings be upon him) said: "Whoever fasts on the day of Arafah, the sins of one past year and one coming year are forgiven for him; and whoever fasts on the day of Ashura, the sins of one year are forgiven for him."
Imam al-Bazzar recorded it. Its chain contains a narrator, Umar bin Suhban, and he is matruk (abandoned).
Imam al-Tabarani recorded it in al-Mu'jam al-Awsat in abridged form, leaving out the day of Ashura, and Imam al-Tabarani's chain is hasan (good).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصيام / 5142