Aayeshah Radiyallahu 'Anha reports:
"The Quraysh observed the fast of 'Aa-shura in the days of jaahiliyyah (pre-Islaarnic period of ignorance). Rasulullah Sallallahu 'Alayhi Wasallam also observed this fast (before the hijrah as a nafl). (After the hijrah) when he came to Madinah Munawwarah he observed these and commanded the ummah also to observe it. When the command to fast in Ramadaan was revealed, it was proclaimed fard, and the fast of 'Aa-shura became nafl. Those who wished, observed them ('Aa-shura) and those who did not, omitted them.
English reference: Book 42, Hadith 292
Arabic reference: Book 43, Hadith 309