الشمائل المحمدية

A’isha said: "‘Ashura' was a day on which Quraish used to fast in the pagan era [al-jahiliyya], and Allah’s Messenger (Allah bless him and give him peace) used to keep it as well. Hence, when he arrived in Medina, he kept its fast and commanded its observance. Then, when Ramadan was decreed, [the fast of] Ramadan became the obligatory religious duty, and 'Ashura' was omitted, so one is free to fast this day or not to fast it.”