Sayyiduna Abdullah bin Umar (may Allah be pleased with him) said regarding the Day of Ashura: The Messenger of Allah (peace and blessings be upon him) himself fasted on this day and also ordered fasting on it, but when the month of Ramadan was made obligatory, fasting on this day was abandoned. Thus, Sayyiduna Ibn Umar (may Allah be pleased with him) would not fast on this day, except if it coincided with a day on which he would customarily fast.
Hadith Referenceالفتح الربانی / كتاب الصيام / 3921
Hadith Gradingمحدثین:صحیح
Hadith Takhrij«اخرجه البخاري: 1892، ومسلم: 2216، (انظر مسند أحمد ترقيم الرسالة: 4483 ترقیم بيت الأفكار الدولية: 4483»
Brief Explanation
Benefits: … In any case, the virtue of fasting on the Day of Ashura remains, but now one should also fast on the 9th of Muharram along with it, as is understood from the resolve of the Prophet sallallahu alayhi wa sallam.
The discussion regarding whether the fast in relation to Ashura is only on the 9th of Muharram or along with it also on the 10th of Muharram is coming ahead.