Musannaf Ibn Abi Shaybah

Hazrat Aisha (may Allah be pleased with her) narrates that the Quraysh used to fast on the day of Ashura during the time of ignorance. When the Messenger of Allah (peace and blessings be upon him) came to Madinah, he fasted on the day of Ashura and ordered the Muslims to fast on that day as well. When the fasting of Ramadan was made obligatory, he said that the fasting of Ramadan is obligatory and the obligation of fasting on Ashura has ended. Whoever wishes may fast, and whoever wishes may leave it.