(19) باب صَوْمِ يَوْمِ عَاشُورَاءَ
Ibn 'Umar (Allah be pleased with them) said that the day of 'Ashura was mentioned before the Messenger of Allah (may peace he upon him). Thereupon the Messenger of Allah, (ﷺ) said:
That was a day on which the people of pre-Islamic days used to observe fast. So he who amongst you likes to observe fast should do so, and he who does not like it should abandon it.
Reference: Sahih Muslim 1126 c
In-book reference: Book 13, Hadith 150
USC-MSA web (English) reference(deprecated numbering scheme): Book 6, Hadith 2506