The Day of Ashura was a day that the people of the pre-Islamic era used to fast. When Ramadan was made obligatory, the Messenger of Allah ﷺ was asked about it, and he said, "It is one of the days of Allah Almighty. Whoever wishes may fast it, and whoever wishes may leave it."
حدثنا يحيى عن عبيد الله أخبرني نافع
كان يوم عاشوراء يوما يصومه أهل الجاهلية فلما نزل رمضان سئل عنه رسول الله ﷺ قال هو يوم من أيام الله تعالى من شاء صامه ومن شاء تركه
Its isnad is sahih. Ubaydullah here is Ibn Umar bin Hafs bin Asim. The meaning of this hadith has already appeared in abbreviated form at 4843, and we noted there that Bukhari and Muslim narrated it at length, as in Muntaqa 2216. This is that lengthy narration.