Sahih Muslim
The Book of Fasting (Kitab Al-Sawm)
Book Number: 006 / Hadith Number: 2510
Abd al-Rahman b. Yazid said: When al-Ash'ath b. Qais entered the house of 'Abdullah he was having his breakfast. He ('Abdullah b. Umar) said: Abd Muhammad (al-Asha'th), come near to the breakfast. Thereupon he said: Is not today the day of 'Ashura? He ('Abd al-Rahman) said: Do you know what the day of 'Ashura is? He said: What is it? He said: It is a day on which the Messenger of Allah (may peace be upon him) used to observe fast before the (fasting) in the month of Ramadan (became) obligatory. But when it became obligatory the (fasting of 'Ashura) was abandoned (as compulsory). Abu Kuraib said: He (the Holy Prophet) abandoned it.