The day of Ashura was a day that the people of the age of ignorance used to fast. When Ramadan was revealed, the Messenger of Allah ﷺ was asked about it, and he said, "It is one of the days of Allah Most High. Whoever wishes may fast it, and whoever wishes may leave it."
حدثنا روح أخبرنا عبيد الله بن الأخنس أخبرني نافع عن عبد الله بن عمر فذكر مثله