What did the Prophet Muhammad (peace be upon him) instruct regarding fasting on the day of 'Ashura' after fasting during Ramadan was prescribed?
 Narrated `Aisha:                     The people used to fast on the day of 'Ashura' before fasting in Ramadan was prescribed but when (the order of compulsory fasting in) Ramadan was revealed, it was up to one to fast on it (i.e. 'Ashura') or not.