Abû Hafsah رضي الله عنه reported that the Prophet صلى الله عليه وسلم said: “Fasting on the day of ‘Arafah absolves the sins for two years: the previous year and the coming year, and fasting on ‘ashûra, (the tenth day of Muharram) atones for the sins of previous years.”

Reported by all except al-Bukhârî and Tirmidhî