Sahih Muslim | Book 6 | Hadith Number 2501

Narrated by 'Aisha (Allah be pleased with her)
'Aisha (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.