Riyad as-Salihin
Riyad as-Salihin 1252
The Excellence of Observing Saum on the Day of 'Arafah, 'Ashura' and Tasu'a (i.e., 9th of Muharram)
Arabic hadith text
وعن أبي قتادة رضي الله عنه أن رسول الله صلى الله عليه وسلم سئل عن صيام يوم عاشوراء فقال: "يكفر السنة الماضية" ((رواه مسلم)).
Translation
Abu Qatadah (May Allah be pleased with him) reported
The Messenger of Allah (ﷺ) was asked about observing As-Saum (the fast) on the tenth day of Muharram, and he replied, "It is an expiation for the sins of the preceding year."[Muslim].