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]. (DE)
App reference (DE): Book 9, Hadith 262 (DE)
Arabic/English book reference (DE): Book 9, Hadith 1252 (DE)