The Prophet ﷺ fasted on Ashura and ordered others to fast it. When Ramadan was made obligatory, this practice was abandoned. Abdullah would not fast it unless it happened to coincide with his regular fasting.
حدثنا مسدد حدثنا إسماعيل عن أيوب عن نافع
صام النبي ﷺ عاشوراء وأمر بصيامه فلما فرض رمضان ترك وكان عبد الله لا يصومه إلا أن يوافق صومه