Abu Qatadah (May Allah be pleased with him) reported:
The Messenger of Allah (Sallallahu Alaihi Wasallam)
was asked about observing As-Saum (the fast)
on the tenth day of Muharram,
and he (Sallallahu Alaihi Wasallam) replied,
"It is an expiation for the sins of the preceding year.''
[Muslim].