Sahih Muslim , Book 13, Hadith 146 (In-Book) ⓘ

'A'isha (Allah be pleased with her) reported that the Messenger of Allah had ordered to observe fast (on 'Ashura) before the fasting in Ramadan was made obligatory. But when it became obligatory, then he who wished fasted on the day of Ashura, and he who wished did not observe it (on that day).

حَدَّثَنَا حَرْمَلَةُ بْنُ يَحْيَى، أَخْبَرَنَا ابْنُ وَهْبٍ، أَخْبَرَنِي يُونُسُ، عَنِ ابْنِ شِهَابٍ، أَخْبَرَنِي عُرْوَةُ بْنُ الزُّبَيْرِ، أَنَّ عَائِشَةَ، - رضى الله عنها - قَالَتْ كَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم يَأْمُرُ بِصِيَامِهِ قَبْلَ أَنْ يُفْرَضَ رَمَضَانُ فَلَمَّا فُرِضَ رَمَضَانُ كَانَ مَنْ شَاءَ صَامَ يَوْمَ عَاشُورَاءَ وَمَنْ شَاءَ أَفْطَرَ ‏.‏