Sahih Muslim Book 6, Hadith Number 2509.

Chapter : Fasting on the day of ‘Ashura (1Oth of Muharram).

‘Abdullah b. Umar (Allah be pleased with them) reported that the day of ‘Ashura was mentioned before the Messenger of Allah (may peace be upon him) 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.