It is narrated from Umm al-Mu'mineen Hazrat Aisha (may Allah be pleased with her) that before the fasting of Ramadan was made obligatory, the Prophet (peace and blessings be upon him) used to order his companions (may Allah be pleased with them) to fast on the tenth of Muharram. Then, when the fasting of the month of Ramadan was made obligatory, the Prophet (peace and blessings be upon him) began to fast only in the month of Ramadan and left the fasting of Ashura. Now, whoever wishes may fast, and whoever wishes may leave it.