Sahih Bukhari · Hadith 4542
Prophetic Commentary on the Qur'an (Tafseer of the Prophet (pbuh)) - كتاب التفسير
حدثنا مسدد، حدثنا يحيى، عن عبيد الله، قال أخبرني نافع، عن ابن عمر رضى الله عنهما قال كان عاشوراء يصومه أهل الجاهلية، فلما نزل رمضان قال " من شاء صامه، ومن شاء لم يصمه ".
Narrated Ibn `Umar: Fasting was observed on the day of 'Ashura' (i.e. 10th of Muharram) by the people of the Pre-lslamic Period. But when (the order of compulsory fasting) in the month of Ramadan was revealed, the Prophet said, "It is up to one to fast on it (i.e. day of 'Ashura') or not."
Gradings are classical jarḥ-wa-taʿdīl, name-matched to a rijāl index (not individually re-verified).