Riyad as-Salihin, Book 9, Hadith 1252 (In-Book) ⓘ
Abu Qatadah (May Allah be pleased with him) reported: The Messenger of Allah 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].
وعن أبي قتادة رضي الله عنه أن رسول الله صلى الله عليه وسلم سئل عن صيام يوم عاشوراء فقال: "يكفر السنة الماضية" ((رواه مسلم)).