(69) باب صِيَامِ يَوْمِ عَاشُورَاءَ
Narrated 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).' "
Reference: Sahih al-Bukhari 2003
In-book reference: Book 30, Hadith 108
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 3, Book 31, Hadith 221