Fasting on the day of Ashura' Sahih Muslim 1126 f
Translation
'Abdullah b. Umar (Allah be pleased with them) reported that the day of 'Ashura was mentioned before the Messenger of Allah (ﷺ) and he said
It is a day when the people in the pre-Islamic days need to observe fast, so he who wishes to observe fast should do so, and he who wishes to abandon it should do so.