(19) باب صَوْمِ يَوْمِ عَاشُورَاءَ
'A'isha (Allah be pleased with her) reported that the Quraish used to fast on the day of Ashura during the pre-Islamic days. The Messenger of Allah (ﷺ) then commanded to fast on that day till (fasting) in Ramadan became obligatory. Then the Messenger of Allah (ﷺ) said:
He who wishes to fast should do so, and he who wishes to break it may do so.
Reference: Sahih Muslim 1125 e
In-book reference: Book 13, Hadith 147
USC-MSA web (English) reference(deprecated numbering scheme): Book 6, Hadith 2503