The Prophet ﷺ commanded us to fast on Ashura before the fast of Ramadan was prescribed. When the fast of Ramadan was prescribed, he neither commanded us nor forbade us from fasting it, but we continued to observe it.
حدثنا وكيع حدثنا سفيان عن سلمة بن كهيل عن القاسم بن مخيمرة عن أبي عمار
أمرنا النبي ﷺ أن نصوم عاشوراء قبل أن ينزل صيام رمضان فلما نزل صيام رمضان لم يأمرنا ولم ينهنا ونحن نفعله
Its isnad is sahih. Qasim bin Mukhaymara Hamdani, Abu Urwa Dimashqi, is trustworthy, virtuous, and agreed upon. The same is true of Abu Ammar Dimashqi, who is Shaddad bin Abdullah Qurashi, their freedman, and he was a freedman of Muawiya.The hadith was narrated by Bukhari 4/244, no. 2002, Fath al-Bari, in fasting, on fasting the day of Ashura, and by Muslim 2/792, no. 1125, in similar wording, and likewise by Abu Dawud 2/326, no. 2442, and Tirmidhi 3/118, no. 753, who graded it sahih, and Ibn Majah 1/533, no. 1737, and Malik 1/299, no. 33, and Darimi 2/37, no. 1763.Something close to it has already preceded at 2154.