Sahih Muslim | Book 6 | Hadith Number 2503

Narrated by 'Aisha (Allah be pleased with her)
'Aisha (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 upon 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.