Sahih Bukhari Volume 002, Book 013, Hadith Number 001.
Narated By Abu Huraira : I heard Allah's Apostle (p.b.u.h) saying, "We (Muslims) are the last (to come) but (will be) the foremost on the Day of Resurrection though the former nations were given the Holy Scriptures before us. And this was their day (Friday) the celebration of which was made compulsory for them but they differed about it. So Allah gave us the guidance for it (Friday) and all the other people are behind us in this respect: the Jews' (holy day is) tomorrow (i.e. Saturday) and the Christians' (is) the day after tomorrow (i.e. Sunday)."