(19) باب صَوْمِ يَوْمِ عَاشُورَاءَ
Jabir b Samura reported that the Messenger of Allah (ﷺ) commanded us to observe fast on the day of Ashura and exhorted us to do it and was particular about it But when (fasting) in Ramadan was made obligatory, he hence. forth neither commanded us nor forbade us, nor was he so particular about it.
Reference: Sahih Muslim 1128
In-book reference: Book 13, Hadith 158
USC-MSA web (English) reference(deprecated numbering scheme): Book 6, Hadith 2514