It is narrated from Hazrat Abu Qatadah (may Allah be pleased with him) that the Noble Prophet (peace and blessings be upon him) said words to the effect that the fast of Arafah is an expiation for two years, and the fast of Ashura is an expiation for one year.
Hadith Referenceمسند احمد / مسند الأنصار رضي الله عنهم / 22616
Hadith Gradingحکم دارالسلام:حديث صحيح، وهذا إسناد ضعيف لاضطرابه والاختلاف فيه