What is the origin of the voluntary fast observed on the day of Ashura after the advent of Islam? (peace be upon him)
 'A'isha (Allah be pleased with her) reported. In the pre-Islamic days fast was observed on the day of Ashura, but with the advent of Islam (its position was ascertained as that of a voluntary fast). Then he who wished to fast fasted, and he who liked to abandon it abandoned it.