(19) باب صَوْمِ يَوْمِ عَاشُورَاءَ
'A'isha (Allah be pleased with her) reported that the Messenger of Allah (ﷺ) had ordered to observe fast (on 'Ashura) before the fasting in Ramadan was made obligatory. But when it became obligatory, then he who wished fasted on the day of Ashura, and he who wished did not observe it (on that day).
Reference: Sahih Muslim 1125 d
In-book reference: Book 13, Hadith 146
USC-MSA web (English) reference(deprecated numbering scheme): Book 6, Hadith 2502