Bukhari Collection: ::Book Number 4:: Hadith # 693
Narrated Abu Huraira:The Prophet said, "We are the last (to come) but we will be the foremost on the Day of Resurrection, nations were given the Book (i.e. Scripture) before us, and we were given the Holy Book after them. This (i.e. Friday) is the day about which they differed. So the next day (i.e. Saturday) was prescribed for the Jews and the day after it (i.e. Sunday) for the Christians. It is incumbent on every Muslim to wash his head and body on a Day (i.e. Friday) (at least) in every seven days."