(19) باب صَوْمِ يَوْمِ عَاشُورَاءَ
Abdullah b. 'Umar (Allah be pleased with both of them) reported that he heard the Messenger of Allah (ﷺ) say about the day of Ashura:
It is a day on which the people of pre-Islamic days observed fast. So he who liked to fast on this day should do so, and he who liked to abandon it should abandon it. 'Abdullah (Allah be pleased with him) did not observe fast except when it coincided (with the days when he was in the habit of observing voluntary fasts during every month).
Reference: Sahih Muslim 1126 d
In-book reference: Book 13, Hadith 151
USC-MSA web (English) reference(deprecated numbering scheme): Book 6, Hadith 2507