Sahih Bukhari Volume 3, Book 30, Hadith 2004

حَدَّثَنَا عَبْدُ اللَّهِ بْنُ مَسْلَمَةَ، عَنْ مَالِكٍ، عَنِ ابْنِ شِهَابٍ، عَنْ حُمَيْدِ بْنِ عَبْدِ الرَّحْمَنِ، أَنَّهُ سَمِعَ مُعَاوِيَةَ بْنَ أَبِي سُفْيَانَ ـ رضى الله عنهما ـ يَوْمَ عَاشُورَاءَ عَامَ حَجَّ عَلَى الْمِنْبَرِ يَقُولُ يَا أَهْلَ الْمَدِينَةِ، أَيْنَ عُلَمَاؤُكُمْ سَمِعْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم يَقُولُ ‏"‏ هَذَا يَوْمُ عَاشُورَاءَ، وَلَمْ يُكْتَبْ عَلَيْكُمْ صِيَامُهُ، وَأَنَا صَائِمٌ، فَمَنْ شَاءَ فَلْيَصُمْ وَمَنْ شَاءَ فَلْيُفْطِرْ ‏"

Narrated By Humaid bin 'Abdur Rahman : That he heard Muawiya bin Abi Sufyan on the day of 'Ashura' during the year he performed the Hajj, saying on the pulpit, "O the people of Medina! Where are your Religious Scholars? I heard Allah's Apostle saying, 'This is the day of 'Ashura'. Allah has not enjoined its fasting on you but I am fasting it. You have the choice either to fast or not to fast (on this day).'"