Sahih Muslim
The Book of Fasting (Kitab Al-Sawm)
Book Number: 006 / Hadith Number: 2509
'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.