Sahih al-Bukhari 4501
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."
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 ‏"‏‏.‏