Sahih Muslim , Book 13, Hadith 145 (In-Book) ⓘ

'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.

حَدَّثَنِي عَمْرٌو النَّاقِدُ، حَدَّثَنَا سُفْيَانُ، عَنِ الزُّهْرِيِّ، عَنْ عُرْوَةَ، عَنْ عَائِشَةَ، - رضى الله عنها - أَنَّ يَوْمَ، عَاشُورَاءَ كَانَ يُصَامُ فِي الْجَاهِلِيَّةِ فَلَمَّا جَاءَ الإِسْلاَمُ مَنْ شَاءَ صَامَهُ وَمَنْ شَاءَ تَرَكَهُ ‏.‏