Sahih Muslim
The Book of Fasting (Kitab Al-Sawm)
Book Number: 006 / Hadith Number: 2503
'A'isha (Allah be pleased with her) reported that the Quraish used to observe fast on the day of Ashura during the pre-Islamic days. The Messenger of Allah (may peace be upog him) then commanded to fast on that day till (fasting) in Ramadan became obligatory. Then the Messenger of Allah (may peace be upon him) said: He who wished to fast should do so. and he who wished to break it may do so.