Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) said: The Messenger of Allah (peace and blessings be upon him) said: "This is the day of Ashura, and the Quraysh used to fast on it during the days of ignorance. So whoever among you wishes may fast on Ashura, and whoever wishes to leave it may leave it." And Sayyiduna Ibn Umar (may Allah be pleased with them both) did not specifically fast on Ashura, except that if Ashura came during the days he was fasting, he would complete his fasts.
Hadith Referenceسنن دارمي / من كتاب الصوم / 1800
Hadith Gradingتحقیق (حسین سلیم أسد الدارانی):رجاله ثقات غير أن ابن إسحاق قد عنعن ولكنه متابع عليه والحديث متفق عليه، [مكتبه الشامله نمبر: 1803]
In the early period of Islam, the fasting of ‘Ashura was prescribed. Then, when the fasting of Ramadan was made obligatory, the fasting of ‘Ashura as an obligation was abrogated, and people were given the choice: whoever wished could fast, and whoever wished could leave it, as is mentioned in al-Bukhari. This narration is coming ahead, but this fast also holds great virtue, as has been mentioned previously.