Sahih Muslim
The Book of Fasting (Kitab Al-Sawm)
Book Number: 006 / Hadith Number: 2499
'A'isha (Allah be pleased with her) reported that the Quraish used to fast on the day of 'Ashura in the pre-Islamic days and the Messenger ot Allah (may peace be upon him) also observed it. When he migrated to Medina, he himself observed this fast and commanded (others) to observe it. But when fasting during the month of Ramadan was made obligatory he said: He who wishes to observe this fast may do so, and he who wishes to abandon it may do so.