Sahih Muslim Book 6, Hadith Number 2504.
Chapter : Fasting on the day of ‘Ashura (1Oth of Muharram).
Abdullah b. ‘Umar (Allah be pleased with them) reported that (the Arabs of) pre-Islamic days used to observe fast on the day of Ashura and the Messenger of Allah (may peace be upon him) observed it and the Muslims too (observed it) before fasting in Ramadan became obligatory. But when it became obligatory, the Messenger of Allah (may peace be upon him) said: ‘Ashura is one of the days of Allah, so he who wished should observe fast and he who wished otherwise should abandon it.