Sahih Muslim | Book 6 | Hadith Number 2523

Narrated by Abu Musa
Abu Musa reported that the people of Khaibar (most of them were Jews) observed fast on the day of 'Ashura and they treated it as 'Id and gave their women ornaments and beautiful dress to wear. The Messenger of Allah (may peace be upon him) said: You (only) observe fast on this day.