Sahih Muslim 2639
صحيح مسلم#2639Sahihحَدَّثَنِي 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 .
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
Scholarly Authentication
The label above is the grade most of these scholars gave — one dissenting opinion no longer speaks for all of them. If any scholar calls a narration fabricated, that verdict is shown instead, whatever the others say. Each scholar's own wording:
Every narration in Sahih Muslim is accepted as Sahih (authentic) by the established status of the book.
In-book reference: 13:145