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].

وعن أبي قتادة رضي الله عنه أن رسول الله ﷺ سئل عن صيام يوم عاشوراء فقال: "يكفر السنة الماضية" ((رواه مسلم)).