Sahih al-Bukhari 4501
Sahih al-Bukhari · Book 65: Prophetic Commentary on the Qur'an (Tafseer of the Prophet (pbuh)) · “O you who believe! Observing As-Saum (the fasting) is prescribed for you as it was prescribed for those before you that you, may become Al- Muttaqun (V.2:183).
Narrated Ibn `Umar:
حَدَّثَنَا مُسَدَّدٌ ، حَدَّثَنَا يَحْيَى ، عَنْ عُبَيْدِ اللَّهِ ، قَالَ أَخْبَرَنِي نَافِعٌ ، عَنِ ابْنِ عُمَرَ ـ رضى الله عنهما ـ قَالَ كَانَ عَاشُورَاءُ يَصُومُهُ أَهْلُ الْجَاهِلِيَّةِ، فَلَمَّا نَزَلَ رَمَضَانُ قَالَ " مَنْ شَاءَ صَامَهُ، وَمَنْ شَاءَ لَمْ يَصُمْهُ " .
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."