Sahih Bukhari Volume 005, Book 058, Hadith Number 278.
Hadith Section/Book: Sahih Bukhari Sahih Bukhari Book 58. Merits of the Helpers in Madina (Ansaar)
Narated By Abu Musa : When the Prophet arrived at Medina, he noticed that some people among the Jews used to respect Ashura' (i.e. 10th of Muharram) and fast on it. The Prophet then said, "We have more right to observe fast on this day." and ordered that fasting should be observed on it.