Sahih Bukhari – Hadith No : 4542

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.”

Hadith In Arabic Text :

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 ‏”‏‏.‏

Chapter : Prophetic Commentary on the Qur’an (Tafseer of the Prophet (pbuh)) – كتاب التفسير