Sahih Muslim
The Book of Fasting (Kitab Al-Sawm)
Book Number: 006 / Hadith Number: 2502
'A'isha (Allah be pleased with her) reported that the Messenger of Allah (may peace be upon him) had ordered to observe fast (on 'Ashura) before the fasting in Ramadan was made obligatory. But when it became obligatory, then he who wished fasted on the day of Ashura, and he who wished did not observe it (on that day).