(19) باب صَوْمِ يَوْمِ عَاشُورَاءَ
'A'isha (Allah be pleased with her) reported. In the pre-Islamic days fast was observed on the day of Ashura, but with the advent of Islam (its position was ascertained as that of a voluntary fast). Then he who wished to fast fasted, and he who liked to abandon it abandoned it.
Reference: Sahih Muslim 1125 c
In-book reference: Book 13, Hadith 145
USC-MSA web (English) reference(deprecated numbering scheme): Book 6, Hadith 2501